The Identifier value element is used within the context of the first three elements to identify the Node. Its data type and format is defined by the IdType.

Identifier values of IdType String are restricted to 4096 characters and shall not contain Unicode control characters. Unicode control characters are defined by Unicode C0 and Unicode C1.

A Node in the AddressSpace shall not have a null NodeId. However, many services defined in OPC 10000-4 define special behaviour if a null NodeId is passed as a parameter.

A canonical null NodeId has an IdType equal to Numeric, a NamespaceIndex equal to 0 and an Identifier equal to 0.

In addition to the canonical null NodeId the alternative values defined in Table 23 shall be considered a null NodeId.

Table 23 – NodeId Alternative Null Values

IdType	NamespaceIndex	Null Value
String	0	A null or Empty String (“”)
Guid	0	A Guid initialised with zeros (e.g. 00000000-0000-0000-0000-000000)
Opaque	0	A null or Empty ByteString