NodeClassesare defined in terms of the Attributesand Referencesthat shall be instantiated (given values) when a Nodeis defined in the AddressSpace. Attributesare discussed in 4.3.3and Referencesin 4.3.4.

Clause 5defines the NodeClassesfor the OPC UA AddressSpace. These NodeClassesare referred to collectively as the metadata for the AddressSpace. Each Nodein the AddressSpaceis an instance of one of these NodeClasses. No other NodeClassesshall be used to define Nodes, and as a result, Clientsand Serversare not allowed to define NodeClassesor extend the definitions of these NodeClasses.