NodeClasses are defined in terms of the Attributes and References that shall be instantiated (given values) when a Node is defined in the AddressSpace. Attributes are discussed in 4.3.3 and References in 4.3.4.
Clause 5 defines the NodeClasses for the OPC UA AddressSpace. These NodeClasses are referred to collectively as the metadata for the AddressSpace. Each Node in the AddressSpace is an instance of one of these NodeClasses. No other NodeClasses shall be used to define Nodes, and as a result, Clients and Servers are not allowed to define NodeClasses or extend the definitions of these NodeClasses.