Use when you need to attach a classification code (Uniclass, OmniClass, MasterFormat, ETIM) or an external library reference to objects in an IFC model, or read and debug the classification and library entities of an existing model. Prevents putting the code in the wrong attribute, inventing IFC entity names, using the IFC2x3 attribute name in an IFC4 file (ItemReference vs Identification, Location vs Specification), and confusing the classification system entity with a single classification code. Covers IfcClassification, IfcClassificationReference, IfcRelAssociatesClassification, the IfcClassificationSelect and IfcClassificationReferenceSelect select types, the library entities IfcRelAssociatesLibrary, IfcLibraryReference and IfcLibraryInformation, the external systems Uniclass 2015 / OmniClass / MasterFormat / ETIM, and the IFC2x3 / IFC4 / IFC4.3 version differences. Keywords: IFC classification, IfcClassification, IfcClassificationReference, IfcRelAssociatesClassification, IfcClassificationSelect, IfcClas
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Use when you need to attach a classification code (Uniclass, OmniClass, MasterFormat, ETIM) or an external library reference to objects in an IFC model, or read and debug the classification and library entities of an existing model. Prevents putting the code in the wrong attribute, inventing IFC entity names, using the IFC2x3 attribute name in an IFC4 file (ItemReference vs Identification, Location vs Specification), and confusing the classification system entity with a single classification code. Covers IfcClassification, IfcClassificationReference, IfcRelAssociatesClassification, the IfcClassificationSelect and IfcClassificationReferenceSelect select types, the library entities IfcRelAssociatesLibrary, IfcLibraryReference and IfcLibraryInformation, the external systems Uniclass 2015 / OmniClass / MasterFormat / ETIM, and the IFC2x3 / IFC4 / IFC4.3 version differences. Keywords: IFC classification, IfcClassification, IfcClassificationReference, IfcRelAssociatesClassification, IfcClassificationSelect, IfcClassificationReferenceSelect, IfcClassificationNotationSelect, IfcLibraryReference, IfcLibraryInformation, IfcRelAssociatesLibrary, IfcRelAssociatesLibrary, Uniclass 2015, OmniClass, MasterFormat, ETIM, classification code, Identification, ItemReference, Location, Specification, ReferenceTokens, ReferencedSource, how do I classify an IFC element, where does the Uniclass code go, classification not showing up, code in wrong field, IFC2x3, IFC4, IFC4.3.
license
MIT
compatibility
Designed for Claude Code. Requires IFC IFC2x3, IFC4, IFC4.3.
metadata
{"author":"OpenAEC-Foundation","version":"1.0"}
IFC Syntax : Classifications
An IFC model links its objects to external classification systems (Uniclass,
OmniClass, MasterFormat, ETIM) and to external libraries through a small, fixed set
of entities. This skill covers those entities, the attribute that carries the actual
code, and the IFC2x3 / IFC4 / IFC4.3 version differences that break round-tripped
files.
Applies to IFC2x3, IFC4, and IFC4.3. Several attribute names and types changed
across versions : see the Version Differences section.
Quick Reference
The three classification entities
IfcClassification the system (Uniclass 2015 as a whole)
^
| ReferencedSource
|
IfcClassificationReference one code (one Uniclass code, e.g. EF_25_10)
^
| RelatingClassification
|
IfcRelAssociatesClassification the link (binds the code to objects)
|
| RelatedObjects
v
IfcWall, IfcWallType, ... the classified objects
ALWAYS use IfcClassification for the system and IfcClassificationReference for a
single code. NEVER attach an IfcClassification directly to elements as if it were a
code : the system entity names the publisher and edition, not one item.
IfcClassification : the system
IfcClassification identifies an external classification system : its publisher,
edition, and name. In IFC4 and IFC4.3 it is a subtype of IfcExternalInformation.
Verified verbatim (IFC4.3) :
ENTITY IfcClassification
SUBTYPE OF (IfcExternalInformation);
Source : OPTIONAL IfcLabel;
Edition : OPTIONAL IfcLabel;
EditionDate : OPTIONAL IfcDate;
Name : IfcLabel;
Description : OPTIONAL IfcText;
Specification : OPTIONAL IfcURIReference;
ReferenceTokens : OPTIONAL LIST [1:?] OF IfcIdentifier;
INVERSE
ClassificationForObjects : SET [0:?] OF IfcRelAssociatesClassification FOR RelatingClassification;
HasReferences : SET [0:?] OF IfcClassificationReference FOR ReferencedSource;
END_ENTITY;
Name is the only required attribute (the system's common name, e.g.
'Uniclass 2015').
Specification is a URI/URN/URL locator of the system. In IFC4 this attribute is
named Location : see Version Differences.
ReferenceTokens lists the delimiter tokens that mark facet boundaries inside a
compound code, so a reader can split EF_25_10_25 into facets.
IfcClassificationReference : one code
IfcClassificationReference is one specific code inside a system. It is a subtype of
, which supplies the attribute that carries the code.
IfcExternalReference
Verified verbatim (IFC4.3) :
ENTITY IfcClassificationReference
SUBTYPE OF (IfcExternalReference);
ReferencedSource : OPTIONAL IfcClassificationReferenceSelect;
Description : OPTIONAL IfcText;
Sort : OPTIONAL IfcIdentifier;
INVERSE
ClassificationRefForObjects : SET [0:?] OF IfcRelAssociatesClassification FOR RelatingClassification;
HasReferences : SET [0:?] OF IfcClassificationReference FOR ReferencedSource;
END_ENTITY;
Inherited from IfcExternalReference (IFC4 and IFC4.3) :
Location (OPTIONAL IfcURIReference) : a URI for the code itself.
Identification (OPTIONAL IfcIdentifier) : the actual code (e.g.
'EF_25_10'). In IFC2x3 this attribute is named ItemReference.
Name (OPTIONAL IfcLabel) : the human-readable title of the classified item.
ALWAYS put the code string in Identification (IFC4+) or ItemReference
(IFC2x3). NEVER put the code in Name : Name is the readable title, not the code.
ReferencedSource points either at the IfcClassification system or at a parent
IfcClassificationReference. Pointing at a parent reference builds a hierarchical
classification tree. Full select details : see references/methods.md.
IfcRelAssociatesClassification : the link
The objectified relationship that binds a classification to objects. Supertype chain
IfcRoot > IfcRelationship > IfcRelAssociates > IfcRelAssociatesClassification.
Verified verbatim (IFC4.3) :
ENTITY IfcRelAssociatesClassification
SUBTYPE OF (IfcRelAssociates);
RelatingClassification : IfcClassificationSelect;
END_ENTITY;
RelatedObjects (SET [1:?] OF IfcDefinitionSelect, inherited from
IfcRelAssociates) : the objects or object types being classified.
RelatingClassification (IfcClassificationSelect) : the
IfcClassificationReference (a code) or the whole IfcClassification (the system)
applied to those objects.
One IfcRelAssociatesClassification applies one classification to many objects at
once. ALWAYS reuse a single relationship for a set of objects that share the same
code. NEVER create one relationship per object when the code is identical.
The library entities (brief)
External library references use a structurally identical mechanism. Verified
verbatim (IFC4.3) :
ENTITY IfcRelAssociatesLibrary
SUBTYPE OF (IfcRelAssociates);
RelatingLibrary : IfcLibrarySelect;
END_ENTITY;
IfcLibrarySelect resolves to IfcLibraryReference (one item in a library) or
IfcLibraryInformation (metadata about the library as a whole). IfcLibraryReference
is a subtype of IfcExternalReference, so it carries Identification exactly like a
classification reference. Full definitions : see references/methods.md.
ALWAYS use the library entities for an external specification or product datasheet
that is not a classification taxonomy. NEVER force a product datasheet into
IfcClassificationReference : a datasheet is a library item, not a taxonomy code.
External classification systems
IfcClassificationReference points into a published system. Common systems :
System
Region
Shape
Example code
Uniclass 2015
UK
Faceted, table-based
EF_25_10_25
OmniClass
North America
Multi-table
Table 21 / Table 23 codes
MasterFormat
North America
Specification numbering
04 20 00
ETIM
Europe
Technical product classes
EC000123
The IfcClassification entity names the system; Identification on the
IfcClassificationReference carries one code from it. ReferenceTokens on the
IfcClassification tells a reader how to split a compound code into facets.
Version differences (compact)
Item
IFC2x3
IFC4
IFC4.3
IfcClassification supertype
none (standalone)
IfcExternalInformation
IfcExternalInformation
IfcClassification system URI attribute
absent
Location
Specification
IfcClassification.Description / ReferenceTokens
absent
present
present
Code attribute on the reference
ItemReference
Identification
Identification
IfcExternalReference.Location type
IfcLabel
IfcURIReference
IfcURIReference
IfcClassificationReference.ReferencedSource type
IfcClassification
IfcClassificationReferenceSelect
IfcClassificationReferenceSelect
IfcRelAssociatesClassification.RelatingClassification type
IfcClassificationNotationSelect
IfcClassificationSelect
IfcClassificationSelect
Hierarchical tree mechanism
IfcClassificationItem tree
nested IfcClassificationReference
nested IfcClassificationReference
Full per-version definitions : see references/methods.md.
Decision Trees
Which entity do I need?
What are you modelling?
|
+-- A whole classification system (publisher, edition, name)
| --> IfcClassification. Set Name (required). Set Specification (IFC4.3) or
| Location (IFC4) to the system URI.
|
+-- One code from a system (e.g. one Uniclass code)
| --> IfcClassificationReference. Put the code in Identification (IFC4+)
| or ItemReference (IFC2x3). Point ReferencedSource at the system.
|
+-- Binding a code (or a system) to objects
| --> IfcRelAssociatesClassification. RelatedObjects = the objects,
| RelatingClassification = the IfcClassificationReference.
|
+-- An external specification, datasheet, or product library item
--> IfcLibraryReference + IfcLibraryInformation, bound by
IfcRelAssociatesLibrary. NOT a classification entity.
Where does the code string go?
You have a code like "EF_25_10". Which attribute?
|
+-- IFC4 / IFC4.3 --> IfcClassificationReference.Identification
|
+-- IFC2x3 --> IfcClassificationReference.ItemReference
|
+-- NEVER --> IfcClassificationReference.Name (Name is the readable
title; the code is not a title)
Flat code or nested tree?
Do you need a classification hierarchy in the file?
|
+-- One code, no parent
| --> IfcClassificationReference.ReferencedSource = the IfcClassification.
|
+-- A child code under a parent code (IFC4 / IFC4.3)
| --> child IfcClassificationReference.ReferencedSource = the parent
| IfcClassificationReference. The parent's ReferencedSource is the
| IfcClassification. This nests because ReferencedSource is typed
| IfcClassificationReferenceSelect.
|
+-- A hierarchy in IFC2x3
--> ReferencedSource on the reference can only point at IfcClassification.
The IFC2x3 tree is built with IfcClassificationItem instead. Do NOT
try to nest IfcClassificationReference in an IFC2x3 file.
Patterns
Pattern : Classify objects with one shared relationship
ALWAYS model a classification as three entities : one IfcClassification (the
system), one IfcClassificationReference per distinct code, and one
IfcRelAssociatesClassification linking that reference to every object that carries
the code.
ALWAYS list every object with the same code in the single relationship's
RelatedObjects set. NEVER emit a separate IfcRelAssociatesClassification per
object when the code is identical : that bloats the file and adds nothing.
ALWAYS classify the type object (IfcWallType) when every occurrence of that type
shares the code; classify the occurrence (IfcWall) only when one element deviates
from its type.
See ifc-core-relationships for the IfcRelAssociates family and
ifc-impl-data-enrichment for the workflow of adding classifications to a model.
Pattern : Put the code in the code attribute
ALWAYS write the classification code string into IfcClassificationReference.Identification
in IFC4 and IFC4.3, or into ItemReference in IFC2x3.
ALWAYS write the human-readable title of the classified item into Name.
NEVER swap the two : a reader extracting codes queries Identification (or
ItemReference), and a code hidden in Name is invisible to that reader.
Pattern : Point the reference at its system
ALWAYS set IfcClassificationReference.ReferencedSource to the IfcClassification
that names the system the code belongs to. A reference with no ReferencedSource is
an orphan code with no system context.
ALWAYS set IfcClassification.Name (the only required attribute) to the system's
common name, such as 'Uniclass 2015'.
Pattern : Build a nested classification tree (IFC4 / IFC4.3)
ALWAYS build a hierarchy by setting a child IfcClassificationReference.ReferencedSource
to its parent IfcClassificationReference, and the top reference's ReferencedSource
to the IfcClassification. This works because ReferencedSource is typed
IfcClassificationReferenceSelect, which accepts both.
NEVER attempt this nesting in IFC2x3 : there ReferencedSource is typed
IfcClassification only, and a hierarchy uses the separate IfcClassificationItem
mechanism.
Pattern : Honour the version differences
ALWAYS write the system URI into IfcClassification.Specification in IFC4.3 and into
IfcClassification.Location in IFC4. The attribute was renamed; using the wrong name
yields an unknown-attribute error.
ALWAYS use Identification for the code in IFC4 and IFC4.3, and ItemReference in
IFC2x3.
NEVER copy an IFC2x3 IfcClassification instance verbatim into an IFC4 file : IFC2x3
IfcClassification is a standalone entity with four attributes, while IFC4 adds
Description, Location, and ReferenceTokens and a supertype. See
ifc-impl-version-migration for the migration procedure.
Pattern : Reference an external library
ALWAYS model an external specification or datasheet with IfcLibraryInformation (the
library as a whole) and IfcLibraryReference (one item in it), bound to objects by
IfcRelAssociatesLibrary.
ALWAYS set IfcLibraryReference.ReferencedLibrary to the owning IfcLibraryInformation
in IFC4 and IFC4.3.
NEVER use a classification entity for a library item : a classification taxonomy and
a reference library are distinct concepts with distinct entities.
Reference Links
references/methods.md : the complete verified entity definitions for IFC2x3,
IFC4, and IFC4.3, the IfcClassificationSelect / IfcClassificationReferenceSelect
/ IfcClassificationNotationSelect / IfcLibrarySelect select types, and the
inherited IfcExternalReference / IfcExternalInformation attributes.
references/examples.md : worked STEP scenarios : a Uniclass system and code, a
shared relationship over many walls, a nested code tree, an IFC2x3 classification,
and a library reference.
references/anti-patterns.md : the failure modes this skill prevents, each with
the reason it fails.
