#### 8.4.3.4 IfcExternalReference

| Référence externe
| Externer Bezug

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.

> HISTORY New entity in IFC2x.

> IFC 2x4 CHANGE Attribute Identification renamed from ItemReference, attribute Location datatype changed, and inverse attribute ExternalReferenceForResources added.

XSD Specification:

<xs:element name="IfcExternalReference" type="ifc:IfcExternalReference" abstract="true" substitutionGroup="ifc:Entity" nillable="true"/>
<xs:complexType name="IfcExternalReference" abstract="true">
<xs:complexContent>
<xs:extension base="ifc:Entity">
<xs:attribute name="Identification" type="ifc:IfcIdentifier" use="optional"/>
</xs:extension>
</xs:complexContent>
</xs:complexType>

EXPRESS Specification:

ENTITY IfcExternalReference

| ABSTRACT SUPERTYPE OF(IfcClassificationReference);

Location | : | OPTIONAL IfcStrippedOptional;
Identification | : | OPTIONAL IfcIdentifier;
Name | : | OPTIONAL IfcStrippedOptional;

| INVERSE

| WHERE

WR1 | : | EXISTS(Identification) OR EXISTS(Location) OR EXISTS(Name);

END_ENTITY;

Attribute Definitions:

Identification | : | 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.
Name | : | 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).

Formal Propositions:

WR1 | : | One of the attributes of IfcExternalReference should have a value assigned.

Inheritance Graph:

ENTITY IfcExternalReference

| ENTITY IfcExternalReference

Location | : | OPTIONAL IfcStrippedOptional;
Identification | : | OPTIONAL IfcIdentifier;
Name | : | OPTIONAL IfcStrippedOptional;

| INVERSE

END_ENTITY;

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