IFC 4.3.x development

8.6.3.2 IfcClassificationReference

Change log

IFC4

additions attributes Description
additions attributes Sort
additions inverses ClassificationRefForObjects
modificationsattributes ReferencedSource definitionChanged from IfcClassification to IfcClassificationReferenceSelect

8.6.3.2.1 Semantic definitions at the entity

An IfcClassificationReference is a reference into a classification system or source (see IfcClassification) for a specific classification key (or notation).

The inherited attributes have the following meaning:

  • Identification: holds the key provided for a specific references to classification items (or tables).
  • Name: allows for a human interpretable designation of a classification notation.
  • Location: optionally holds a direct URI link into the classification system (or source) to hyperlink the classification key.

The IfcClassificationReference can either be assigned directly to the IfcClassification, such as if no classification hierarchy has to be included, or it references the parent classification notation, if the fully classification hierarchy is included in the data set. The first is refered to as "lightweight classification", and the second as "full classification". The attribute ReferencedSource then holds the following information (choice by IfcClassificationReferenceSelect):

  1. being of type IfcClassification: direct reference to the classification system (with meta information provided);
  2. being of type IfcClassificationReference: reference to the parent classification notation within the classification hierarchy.

EXAMPLE  The IfcClassificationReference can be used as a form of 'lightweight' classification through the 'Identification' attribute inherited from the abstract IfcExternalReference class. In this case, the 'Identification' could take (for instance) the Uniclass notation "L6814" which, if the classification was well understood by all parties and was known to be taken from a particular classification source, would be sufficient. The Name attribute could be the title "Tanking". This would remove the need for the overhead of the more complete classification structure of the model.

HISTORY  New entity in IFC2x.

IFC4 CHANGE  The attribute Description and inverse attribute HasReferences are added. The attribute Identification has been renamed from ItemReference.

8.6.3.2.1 Attributes

# Attribute Type Description
IfcExternalReference
1 Location OPTIONAL IfcURIReference Location, where the external source (classification, document or library) can be accessed by electronic means. The electronic location is provided as an URI, and would normally be given as an URL location string.

> IFC4 CHANGE  The data type has been changed from IfcLabel to IfcURIReference

> .
2 Identification OPTIONAL IfcIdentifier The Identification provides a unique identifier of the referenced item within the external source (classification, document or library). It may be provided as

* a key, e.g. a classification notation, like NF2.3

* a handle

* a uuid or guid

It may be human readable (such as a key) or not (such as a handle or uuid) depending on the context of its usage (which has to be determined by local agreement).

> IFC4 CHANGE Attribute renamed from ItemReference for consistency.

>
3 Name OPTIONAL IfcLabel Optional name to further specify the reference. It can provide a human readable identifier (which does not necessarily need to have a counterpart in the internal structure of the document).
ExternalReferenceForResourcesSET [0:?] OF IfcExternalReferenceRelationship FOR RelatingReference Reference to all associations between this external reference and objects within the IfcResourceObjectSelect that are tagged by the external reference.

> IFC4 CHANGE  New inverse attribute added with upward compatibility.

>
IfcClassificationReference
4 ReferencedSource OPTIONAL IfcClassificationReferenceSelect The classification system or source that is referenced.

> IFC4 CHANGE  Data type changed to IfcClassificationReferenceSelect.
5 Description OPTIONAL IfcText Description of the classification reference for informational purposes.

> IFC4 CHANGE  New attribute added at the end of the attribute list.
6 Sort OPTIONAL IfcIdentifier Optional identifier to sort the set of classification references within the referenced source (either a classification facet of higher level, or the classification system itself).

> IFC4 CHANGE  New attribute added at the end of the attribute list.
ClassificationRefForObjects SET [0:?] OF IfcRelAssociatesClassification FOR RelatingClassificationThe classification reference with which objects are associated.

> IFC4 CHANGE  New inverse attribute.
HasReferences SET [0:?] OF IfcClassificationReference FOR ReferencedSource The parent classification references to which this child classification reference applies. It can either be the final classification item leaf node, or an intermediate classification item.

> IFC4 CHANGE New inverse attribute.

8.6.3.2.2 Entity inheritance

dot_inheritance IfcClassificationReference IfcClassificationReference IfcExternalReference IfcExternalReference IfcClassificationReference->IfcExternalReference IfcExternalReference_children 5 more... IfcExternalReference_children->IfcExternalReference

8.6.3.2.3 Formal representations

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;