Referencesare used to relate Nodesto each other. They can be accessed using the browsing and querying Servicesdefined in OPC 10000-4.

Like Attributes, they are defined as fundamental components of Nodes. Unlike Attributes, Referencesare defined as instances of ReferenceType Nodes. ReferenceType Nodesare visible in the AddressSpace and are defined using the ReferenceType NodeClass(see 5.3).

The Nodethat contains the Referenceis referred to as the SourceNodeand the Nodethat is referenced is referred to as the TargetNode. The combination of the SourceNode, the ReferenceTypeand the TargetNodeare used in OPC UA Servicesto uniquely identify References. Thus, each Nodecan reference another Nodewith the same ReferenceTypeonly once. Any subtypes of concrete ReferenceTypesare considered to be equal to the base concrete ReferenceTypeswhen identifying References(see 5.3for subtypes of ReferenceTypes). Figure 4illustrates this model of a Reference.

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Figure 4– Reference Model

The TargetNodeof a Referencemay be in the same AddressSpaceor in the AddressSpaceof another OPC UA Server. TargetNodeslocated in other Serversare identified in OPC UA Servicesusing a combination of the remote Servername and the identifier assigned to the Nodeby the remote Server.

OPC UA does not require that the TargetNode exists, thus Referencesmay point to a Nodethat does not exist.