OPC UA relates Nodesusing References. This concept is intentionally a simple construct, where the only information of a Referenceis the SourceNode, the TargetNode, the ReferenceTypeand for asymmetric ReferenceTypesthe direction (forward or inverse).

Sometimes, it is desirable to add more information to such a Reference. The concept of a ReferenceDescriptionkeeps the simple Referenceunchanged, and creates a Variablewhich is used to add more information about the Reference. Such a Variablecan be referenced from the SourceNodeand / or the TargetNodeof such a Reference. Potentially, it can also be referenced by other Nodesto provide additional information (see also 5.2). As Nodescan have several References, each ReferenceDescription Variablecontains the unique definition of one Reference. An example is given in Figure 2. ObjectA references ObjectB and ObjectC, each using the ReferenceTypeSomeReferenceType. Both Referencesare described by a ReferenceDescription, that is, the VariablesSomeReferenceType1 and SomeReferenceType2. The Valuesof those Variablesuniquely identify the Referencesby providing SourceNode, TargetNode, ReferenceTypeand direction. In the example, both ReferenceDescriptionshave a Propertywith additional information, one called AdditionalInformationABC and the other AdditionalInformationXYZ.

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Figure 2– Example of ReferenceDescriptions

The following rules apply for using a ReferenceDescription.