# 8.6.3.7 IfcExternalReference

ABSTRACT This definition may not be instantiated

## 8.6.3.7.1 Semantic definition

An IfcExternalReference is the identification of information that is not explicitly represented in the current model or in the project database (as an implementation of the current model). Such information may be contained in classifications, documents or libraries. The IfcExternalReference identifies a particular item, such as a dictionary entry, a classification notation, or a document reference within the external source.

Only the Location (as a URL) is given to describe the place where the information can be found. Also an optional Identification as a key to allow more specific references (as to sections or tables) is provided. The Identification defines a system interpretable method to identify the relevant part of information at the source. In addition a human interpretable Name can be assigned to identify the information subject, such as a classification code.

IfcExternalReference is an abstract supertype of all external reference entities.

## 8.6.3.7.5 Formal representation

ENTITY IfcExternalReference
ABSTRACT SUPERTYPE OF (ONEOF
(IfcClassificationReference
,IfcDocumentReference
,IfcExternallyDefinedHatchStyle
,IfcExternallyDefinedSurfaceStyle
,IfcExternallyDefinedTextFont
,IfcLibraryReference));
Location : OPTIONAL IfcURIReference;
Identification : OPTIONAL IfcIdentifier;
Name : OPTIONAL IfcLabel;
INVERSE
ExternalReferenceForResources : SET [0:?] OF IfcExternalReferenceRelationship FOR RelatingReference;
WHERE
WR1 : EXISTS(Identification) OR EXISTS(Location) OR EXISTS(Name);
END_ENTITY;