classification-facet.md

Classification facet

A Classification System is a defined hierarchy to categorise elements. Some popular classification systems include "Uniclass 2015", "ETIM" and "CCI". Within a System, there is a hierarchy of short reference Codes that categorise elements in increasing levels of specificity, such as "EF_25_10" and "EF_25_10_25". Any object in IFC model can have a Classification Reference.

The Classification Facet is different to the Entity Facet. The Entity Facet is restricted to built-in IFC classes and predefined types, which may also function as a method of Classification. In contrast, a classification refers to a third party, non-IFC classifications.

IFC models keep track of classification names, dates, versions, and other data to uniquely identify them. For this reason, Classification requirements should use the Classification Facet, as opposed to the Property Facet.

Classifications are a great way to identify Applicable entities, or Require that entities should follow a nominated Classification system by a workflow, such as in an asset management system, work breakdown structure, or coordination requirement.

Parameters

Parameter	Required	Restrictions Allowed	Meaning
System	✔️	✔️	The name of the classification system.
Value	❌	✔️	The value of a refeference code within that classification system. It is typically an official name or a short code.
URI	❌	❌	Uniform Resource Identifier of the class. Used to reference a standardized definition of a class, to ensure consistency of interpretation. The target resource should include a name and definition, and preferably comply with the ISO 12006-3 and ISO 23386. This is an optional attribute that is not subject to IDS checking - the IFC model does not need to have the same or any URI. One source of valid URIs is the bSDD, and an example URI is that of a "Beam": https://identifier.buildingsmart.org/uri/buildingsmart/ifc/4.3/class/IfcBeam.

Classification facet interpretation

Applicability

Classification System	Classification Value	IDS Interpretation
ETIM	-	Applies to all entities classified using the ETIM classification system, regardless of the value.
ETIM	EC000009	Applies to all entities with a code EC000009 of classification system ETIM.

Requirements

IDS Cardinality	Classification System	Classification Value	URI	Configuration Allowed?	IDS Interpretation
REQUIRED	ETIM	-	https://id...	✅	Applicable objects must have the classification system ETIM populated (i.e. not null).
REQUIRED	ETIM	EC000009	https://id...	✅	The classification ETIM must have the value EC000009 (on applicable objects).
OPTIONAL	ETIM	-	https://id...	❌	Not allowed. Optionality does not make sense - no added field to require.
OPTIONAL	ETIM	EC000009	https://id...	✅	If the classification ETIM exists on applicable objects, it needs to have the value EC000009.
PROHIBITED	ETIM	-	https://id...	✅	The classification ETIM must not exist on applicable objects, even if empty.
PROHIBITED	ETIM	EC000009	https://id...	✅	The classification ETIM must not have the value EC000009 (on applicable objects). Null is also an allowed value in this case.