OPC UA Servers shall provide type definitions for Objects and Variables. The HasTypeDefinition Reference shall be used to link an instance with its type definition represented by a TypeDefinitionNode. Type definitions are required; however, OPC 10000-5 defines a BaseObjectType, a PropertyType, and a BaseDataVariableType so a Server can use such a base type if no more specialised type information is available. Objects and Variables inherit the Attributes specified by their TypeDefinitionNode (see 6.4 for details).
In some cases, the NodeId used by the HasTypeDefinition Reference will be well-known to Clients and Servers. Organizations may define TypeDefinitionNodes that are well-known in the industry. Well-known NodeIds of TypeDefinitionNodes provide for commonality across OPC UA Servers and allow Clients to interpret the TypeDefinitionNode without having to read it from the Server. Therefore, Servers may use well-known NodeIds without representing the corresponding TypeDefinitionNodes in their AddressSpace. However, the TypeDefinitionNodes shall be provided for generic Clients. These TypeDefinitionNodes may exist in another Server.
The following example, illustrated in Figure 5, describes the use of the HasTypeDefinition Reference. In this example, a setpoint parameter “SP” is represented as a DataVariable in the AddressSpace. This DataVariable is part of an Object not shown in the figure.
To provide for a common setpoint definition that can be used by other Objects, a specialised VariableType is used. Each setpoint DataVariable that uses this common definition will have a HasTypeDefinition Reference that identifies the common “SetPoint” VariableType.