Revit MEP tagging glossary: BOP, invert, ITM, tag families, and more

These are the terms that come up when you model, coordinate, and tag MEPF shop drawings in Revit. Each one has a short, plain definition, and where we have a guide on the topic, a link to it. The first five get a longer explanation because they cause the most confusion.

Pipe and duct elevations

BOP elevation

BOP means bottom of pipe. It is the height of the outer bottom surface of the pipe, measured from a level or the project datum. Installers read it to set hangers, because the hanger sits under the pipe.

In Revit, BOP is the Bottom Elevation parameter. A pipe has one at each end, Upper End Bottom Elevation and Lower End Bottom Elevation. On a level pipe the two are the same. On a sloped pipe they differ, so a tag has to say which end it means. Revit also has a spot-based bottom elevation, the value at one point along the pipe, which can be tagged but not scheduled.

To show BOP on a sheet, add the bottom elevation parameter to a pipe tag's label and put a "BOP" prefix in front of it, so the reader knows the datum. You can also place a Spot Elevation set to show the bottom.

BOP is not the same as invert. Bottom of pipe is the outside of the wall, invert is the inside. See how to tag pipe elevation.

Invert elevation

Invert elevation is the height of the inside bottom of a pipe, the lowest point the fluid touches. It sits above BOP by exactly the wall thickness.

Gravity systems, such as sanitary, storm, and condensate drainage, are designed to invert levels, because the invert sets the fall. A drainage standard that works to inverts needs the Invert Elevation parameter in the tag, not Bottom Elevation. Using the wrong one moves every number on the sheet by the wall thickness.

Revit gives a pipe an invert value at each end, plus a spot-based one. The opposite of the invert, the inside top of the pipe, is called the obvert.

For gravity pipes, label each end, or label the invert at the points the design fixes, such as a connection or a manhole. See how to tag pipe elevation and pipe slope tags.

CL (centerline elevation)

The height of the center of a pipe or duct. Common on pressure piping, where the center is what lines up with equipment connections.

TOP (top of pipe)

The height of the outer top surface of a pipe. Used where the top matters, such as clearance under a beam.

Spot elevation

A Revit annotation that shows the elevation at one point you click. Placed from Annotate, Dimension, Spot Elevation. It reads the element at that point, so on a sloped pipe the value depends on where you click. See spot elevations for MEP.

Obvert

The height of the inside top of a pipe. The opposite of the invert.

BOD and TOD

Bottom of duct and top of duct. The heights of the outer bottom and top surfaces of a duct. In Revit they are the duct's Bottom Elevation and Top Elevation parameters. See duct elevation tags.

Middle elevation

The height of the center of a duct or pipe in Revit. It was called Offset before Revit 2019.

Slope

The fall of a pipe over its length, given as a ratio, a percentage, or inches per foot. Gravity drainage needs it, and the elevation at each end differs because of it. See pipe slope tags.

Level

A horizontal datum in Revit, such as Level 1 or Roof, that elements are placed relative to. Elevations on MEP tags are usually measured from the element's reference level or from the project datum.

Tags and annotation

Tag family

A tag family is an annotation family, a .rfa file, that shows information about an element on a view. Each tag family belongs to one tag category, such as Pipe Tags, Duct Tags, or Mechanical Equipment Tags, and can only tag elements of that category.

Inside the family, a label reads parameters from the element, such as size, system, or elevation. The tag does not store the text. It reads it, so when the element changes, the tag changes with it. A question mark on a tag means the label is reading a parameter that is empty or missing on that element.

A family can hold several types, for example a size-only type and a size-and-elevation type. You switch between them in the Type Selector without editing the family.

Most firms keep their own tag families with their own text size, prefixes, and layout, and those families are a large part of the firm's drawing standard. RayCAD uses your tag families as they are and does not ship its own. See tag families and firm tagging standards.

Shared parameter

A shared parameter is a custom parameter defined in a shared parameter file, a text file that many projects and families can use. Each one has a fixed ID called a GUID.

The GUID is what makes it work in tags. A tag label can read built-in parameters and shared parameters. It cannot read a plain family parameter or a project parameter that was typed in by hand. So when a firm wants a custom value on a tag, such as a damper's fire rating or an equipment ID, the value has to live in a shared parameter.

The GUID has to match exactly. A project parameter with the same name but a different GUID shows as a question mark in the tag. The same happens with linked models, where the host's tag family and the linked model must use the same shared parameter definition.

Shared parameters are also what schedules group and filter on. RayCAD reads your shared parameters through your own tag families, with no restriction. See why a tag is not showing and information-based tagging.

Label parameter

A parameter placed inside a tag family's label. It decides what text the tag shows. Edited with Edit Label in the family editor.

Tag All Not Tagged

The Revit command at Annotate, Tag, Tag All. It places one tag on every untagged element of the chosen categories in the view. It does not decide which elements need a tag or where tags should go, so tags often overlap. See why Tag All is not working.

Tag by Category

The Revit command that tags one element at a time as you click it, using the loaded tag for that element's category.

Tag leader

The line from a tag to the element it describes. It can be on or off, with an attached end that stays locked to the element or a free end you place. See tag leaders.

Host element

The element a tag is attached to and reads its values from. If the host is deleted, the tag goes with it.

Annotation

Anything added to a view to explain the model rather than model it: tags, text, dimensions, keynotes, symbols, and revision clouds. Annotation belongs to one view. See annotation automation.

Mark and Type Mark

Two built-in Revit parameters for identifying elements. Mark is set per instance, such as AHU-01. Type Mark is shared by every element of a type. A tag that shows the same value on every element is usually reading Type Mark.

Multi-category tag

A tag family that can tag elements of more than one category, using parameters those categories share.

Keynote

A short code on a view, such as 23 31 13.A, that points to a note in a keynote legend. Used to keep long notes off the drawing.

Text note

Free text placed on a view. Unlike a tag, it does not read anything from the model, so it does not update when the model changes.

Dimension

An annotation that shows the distance between elements or references, such as a duct from a gridline.

Annotation scale

Tags, text, and symbols are drawn at the size they print. When the view scale changes, they keep their printed size while the model gets bigger or smaller around them. This is why a tag that fits at 1:50 can crowd a view at 1:100.

Revision cloud

A cloud drawn around a changed area of a sheet, tied to a revision in the sheet's revision schedule.

Tag overlap

A tag sitting on another tag, on an element, or on a dimension, which makes the sheet hard to read. The most common reason a sheet goes back for cleanup. See overlapping tags.

Tagging standard

A firm's rules for tags: which elements get one, what each tag shows, text size, leaders, spacing, and orientation. Often written down in a drafting standard or BIM execution plan. See firm standards.

Auto-tagging

Placing tags with software instead of by hand. Ranges from Revit's Tag All, which tags everything in a category, to tools that decide which elements need a tag and where each one goes. RayCAD is an auto-tagging plugin for Revit. See information-based tagging and intelligent placement.

Revit basics

Family

The building block of a Revit model. Every element, from a duct to a tag, comes from a family. There are three kinds: system, loadable, and in-place.

System family

A family built into Revit and saved in the project, such as ducts, pipes, walls, and levels. You cannot load it from a file, only edit its types.

Loadable family

A family saved as its own .rfa file and loaded into projects, such as air terminals, equipment, fittings, and tags.

In-place family

A one-off element modeled inside the project. Firms avoid them, because they are hard to reuse, tag, and schedule.

Type and instance parameters

A type parameter is shared by every element of a type, such as a fan coil's capacity. An instance parameter belongs to one element, such as its Mark or elevation.

Built-in parameter

A parameter Revit defines itself, such as Size, System Type, or Bottom Elevation. Tags can read it with no setup.

Project parameter

A custom parameter added to categories in one project. It can be scheduled, but a tag can only read it if it is also a shared parameter.

Category

The class an element belongs to, such as Ducts, Pipes, or Air Terminals. Tags, visibility, and schedules all work by category.

Project template

A .rte file a firm starts every project from. It carries the firm's view templates, tag families, line styles, and settings.

Visibility/Graphics

The Revit dialog, opened with VG, that controls which categories, subcategories, and annotation categories show in a view and how they look.

Worksharing

The Revit setup that lets several people work on one model at once. Everyone works on a local copy and syncs it with a central model.

Workset

A named group of elements in a workshared model, such as HVAC or Plumbing. Worksets control who can edit what and what loads.

Phase

A stage of the project, such as Existing or New Construction, that elements belong to. Views show elements by phase.

Design option

A way to keep several versions of part of a design in one model, so you can compare them.

Add-in

A program that runs inside Revit and adds commands, also called a plugin. Built on the Revit API. RayCAD is a Revit add-in. See Revit integration.

Revit API

The programming interface Autodesk provides for writing add-ins in .NET languages such as C#. See building a tagging add-in.

Dynamo

A visual programming tool that ships with Revit. You connect nodes to automate tasks without writing a full add-in.

Macro

A small program written and run inside Revit's Macro Manager. See a Revit macro for Tag All.

Revit LT

A lower-priced edition of Revit with fewer features. It does not support add-ins, so tools like RayCAD cannot run in it.

Views and sheets

View template

A saved set of view settings, such as scale, detail level, and which categories and tags are visible, applied to many views at once. A tag that will not appear is often hidden by the view template.

Linked model

Another Revit model loaded into the current one as a reference, such as the architect's model inside an MEP model. Elements in a link can be tagged, with some limits. See tagging a linked model.

Floor plan

A view looking down at a horizontal cut through the model. Most MEP tagging happens here.

Reflected ceiling plan

A view of the ceiling as if reflected in a mirror on the floor, often shortened to RCP. Air terminals, lights, and sprinkler heads are usually shown here.

Section view

A view of a vertical cut through the model. Used to show the heights of ducts and pipes and the clearances between them.

Elevation view

A flat view of the model from the side, such as the north face of a plant room.

3D view

A view of the model in three dimensions, used for checking and for isometric shop drawings.

View range

The settings that decide what a plan view shows: the cut plane, the top and bottom of the view, and the view depth. An element outside the view range does not show, and neither does its tag.

Detail level

Coarse, Medium, or Fine. Controls how much detail elements show in a view. Ducts and pipes show as single lines at Coarse and double lines at Medium and Fine.

Crop region

The boundary that limits how much of the model a view shows.

Scope box

A box that sets the extents of crop regions and datums, such as grids and levels, across many views at once. Used to split a large floor into areas.

Dependent view

A view that shares its parent's settings but shows only part of it, usually one area of a large floor plan.

Sheet

A drawing page in Revit. Views are placed on sheets, and sheets are what get printed and issued.

Viewport

The window a view appears in on a sheet. One view can be on one sheet only.

Title block

The border and information panel on a sheet: project name, sheet number, revisions, and stamps.

Schedule

A table view that lists elements and their parameters, such as an equipment schedule. It updates with the model.

Legend

A view that explains symbols and line types, such as what each duct system color means.

MEP systems and modeling

System classification

The kind of system a duct or pipe carries, such as Supply Air, Return Air, Exhaust Air, Hydronic Supply, Sanitary, or Domestic Cold Water. Built into Revit and fixed.

System type

A named system a firm defines under a classification, such as Chilled Water Supply or Kitchen Exhaust. It sets the system's color, abbreviation, and materials, and tags often show its abbreviation.

Routing preferences

The settings on a pipe or duct type that decide which fittings Revit uses as you model, such as elbows, tees, and transitions, by size range.

Connector

The point on a family where a duct or pipe attaches, such as the inlet on a diffuser. Connectors carry flow, size, and system information.

Fitting

A part that joins ducts or pipes: an elbow, tee, transition, reducer, takeoff, or cap. See pipe fitting and accessory tags.

Accessory

A part placed inline on a duct or pipe that is not a fitting, such as a damper, valve, strainer, or flex connector.

Insulation and lining

Insulation wraps the outside of a duct or pipe. Lining sits inside a duct. Both are separate elements in Revit and can be tagged. See insulation tags.

Riser

A pipe or duct that runs vertically between floors. Risers show as a symbol in plan and are tagged where they enter and leave a floor.

Rise and drop

The symbol in plan where a duct or pipe goes up or down. Changes in elevation are where elevation tags are needed.

Placeholder

A single-line duct or pipe in Revit used early in design, before sizes and fittings are known. It is converted to real ducts and pipes later.

Hydronic piping

Pipes that carry water for heating or cooling, such as chilled water and heating hot water.

Domestic water

The building's drinking water supply, hot and cold.

Sanitary and storm drainage

Gravity pipes that carry wastewater and rainwater. They are set out to invert levels and slope.

Fabrication

ITM

An ITM file is an Autodesk Fabrication item, the definition of one fabrication part: its shape, sizes, connectors, material, and costs. ITMs come from the Fabrication products, CADmep, ESTmep, and CAMduct, and are grouped into a fabrication configuration, sometimes called the database.

Revit does not use ITM files on their own. You load a fabrication configuration into the project, and its items become MEP fabrication parts you can place. Because each part comes from a real manufacturer's item, the model matches what the shop builds, which is why fabrication models drive shop drawings that people cut metal from.

Each fabrication part carries the item's values, such as Item Number. Revit ships no ready-made fabrication tags, so firms build their own from a generic tag set to a fabrication tag category, such as MEP Fabrication Ductwork Tags. Some item values are not available to a tag. See how to tag fabrication parts.

RayCAD tags fabrication ductwork: straights, fittings, and accessories. It does not tag fabrication pipework, hangers, or spools today. See fabrication tagging.

MEP fabrication part

A Revit element made from a fabrication configuration item, rather than from a regular Revit family. Has its own categories, such as MEP Fabrication Ductwork, MEP Fabrication Pipework, and MEP Fabrication Hangers.

Spool

A group of pipe parts built together in the shop and shipped to site as one piece. Spool drawings show each spool with its parts and dimensions.

Hanger

A support that holds a pipe or duct from the structure above. Hangers are placed using the bottom elevation of what they hold.

Fabrication configuration

The set of ITM items and settings a firm uses for fabrication, also called the fabrication database. Loaded into Revit to place fabrication parts.

CADmep

Autodesk's fabrication tool that runs in AutoCAD. ESTmep is its estimating tool and CAMduct drives the cutting machines. All three share ITM items with Revit.

Item number

A fabrication part's ID number, used to match the part on the drawing with the part made in the shop.

BIM and coordination

BIM

Building information modeling. Designing and building from a shared 3D model in which every element carries information, such as size, system, and maker.

Federated model

One model combined from each trade's separate models, used for coordination.

Clash detection

Finding places where elements from different trades occupy the same space, such as a duct running through a beam. Usually run in Navisworks, Revizto, or Autodesk Construction Cloud.

Hard clash

Two elements physically intersecting.

Soft clash

An element entering another's clearance zone, such as the access space in front of a panel or around a valve, without touching it.

Clash resolution

Fixing clashes by moving, resizing, or rerouting elements, usually agreed trade by trade in coordination meetings. Every reroute changes the views, so tags need updating after each round. See updating tags after model changes.

Clearance zone

Space kept free around equipment and services for access, maintenance, or code.

Navisworks

Autodesk's software for combining and reviewing models. Its Clash Detective tool runs clash tests.

Revizto

A coordination platform for sharing models, tracking issues, and running clash checks across teams.

Autodesk Construction Cloud

Autodesk's online platform for sharing models and documents and running coordination, often called ACC. It replaced BIM 360.

IFC

Industry Foundation Classes. An open file format for exchanging BIM models between different software. Not to be confused with issued for construction.

BCF

BIM Collaboration Format. An open format for sending issues, such as clashes, between tools, with a viewpoint and a comment.

Common data environment

The single shared place where a project's models and documents live, often shortened to CDE.

Point cloud

Millions of measured points from a laser scan of an existing building, loaded into Revit as a reference.

Scan to BIM

Building a model from a point cloud, usually for renovation work.

Sleeve and penetration

A penetration is where a duct or pipe passes through a wall, floor, or beam. A sleeve is the opening or tube placed to allow it. Penetration drawings show where each one goes.

LOD

Level of Development. How complete and reliable a model element is, from LOD 100, a rough placeholder, to LOD 400, ready to fabricate. LOD 300 is precise design geometry, and LOD 350 adds how elements connect to other trades.

BIM coordinator

The person who runs clash detection and coordination meetings and keeps the trades' models aligned.

BIM manager

The person who sets a firm's or project's BIM standards, templates, and workflows.

Drawings and delivery

Shop drawing

A drawing made by the contractor or fabricator that shows exactly how something will be built and installed, with sizes, elevations, and tags. It is more detailed than the design drawings. See MEP shop drawings in Revit and HVAC shop drawings.

Workshop drawing

The name for a shop drawing in the UK, the Middle East, Australia, and parts of Asia. Same thing.

Coordination drawing

A drawing that combines the trades, such as ducts, pipes, and sprinklers, in one view, to show they fit together without clashes before installation.

Construction documents

The drawings and specifications issued for building, usually by the design team. Shop drawings are made from them.

LOD 400

Level of Development 400. A model element at this level is detailed enough to fabricate and install from. Shop drawing models usually aim for it.

BIM execution plan

A document, often called a BEP, that sets how a project uses BIM: who models what, to what level, in which software, and how files are shared.

MEPF

Mechanical, electrical, plumbing, and fire protection. The building services trades.

VDC

Virtual design and construction. Using models to plan, coordinate, and sequence the build before work starts on site. Many contractors have a VDC team that owns the shop drawings.

RFI

Request for information. A formal question from the contractor to the design team when something on the drawings is missing or unclear. Missing tags are a common cause. See fewer RFIs.

Submittal

A package the contractor sends the design team for approval before building, such as shop drawings, product data, and samples.

Markup

Comments drawn on a sheet by a reviewer, also called redlines. Usually done in Bluebeam or a PDF reader.

Issued for construction

The drawing status that means a set is approved to build from, often shortened to IFC. Not to be confused with the file format.

Record drawings

Drawings updated after construction to show what was actually built, also called as-builts.

Specification

The written part of the contract documents, with requirements for materials, products, and workmanship. Often organized by MasterFormat divisions, such as Division 23 for HVAC.

Change order

A formal change to the contract scope, cost, or schedule after work has started.

Sheet QA

Checking a sheet before issue for missing tags, overlaps, wrong values, and standard problems. See the MEPF sheet QA checklist.

Projects and roles

AEC

Architecture, engineering, and construction. The industry as a whole.

MEP engineer

The consultant who designs the building's MEP systems and issues the design drawings.

MEP contractor

The company that builds and installs the MEP systems. Most shop drawings are made by MEP contractors or for them.

Mechanical contractor

An MEP contractor that installs HVAC and often piping.

General contractor

The company that runs the whole construction project and hires the trade contractors.

BIM drafting service

A firm that models, coordinates, and produces shop drawings for contractors, often from another country.

Design-build

A project where one company is hired to both design and build.

Design-bid-build

A project where the design is finished first, then contractors bid to build it.

Equipment and components

Air terminal

A diffuser, grille, or register where air enters or leaves a room. See how to tag air terminals.

VAV

Variable air volume box. A unit in the ductwork that changes the airflow to one zone to control its temperature.

AHU

Air handling unit. The large unit that filters, heats or cools, and moves air through the ductwork. See how to tag mechanical equipment.

FCU

Fan coil unit. A small unit with a fan and a coil that heats or cools one room or zone.

Damper

A plate in a duct that controls or stops airflow. Fire and smoke dampers close in a fire. In Revit, dampers are duct accessories. See duct accessory tags.

Flex duct

Flexible duct, usually the short run from a rigid duct to an air terminal. Revit has a separate flex duct category with its own tags. See flex duct tags.

Cable tray

An open tray that carries electrical cables along a route. RayCAD's electrical tagging is coming soon. See conduit and cable tray tagging.

Conduit

A tube that protects and routes electrical wiring.

Sprinkler head

The outlet on a fire sprinkler pipe that sprays water when it detects heat. RayCAD's fire protection tagging is coming soon. See fire sprinkler tagging.

Diffuser, grille, and register

Kinds of air terminal. A diffuser spreads supply air, a grille covers an opening with no damper, and a register is a grille with a damper.

Exhaust fan

A fan that pulls air out of a space, such as a toilet or kitchen, and sends it outside.

Chiller

The machine that makes chilled water for cooling.

Boiler

The machine that makes hot water or steam for heating.

Cooling tower

The unit on the roof that rejects heat from a chiller's condenser water to the outside air.

Pump

Moves water through hydronic and domestic water systems. Tagged as mechanical equipment.

Valve

A pipe accessory that starts, stops, or controls flow, such as a ball, gate, check, or balancing valve.

Plumbing fixture

A sink, toilet, urinal, shower, or drain. See plumbing fixture tags.