Nodes are unambiguously identified using a constructed identifier called the NodeId. Some Servers may accept alternative NodeIds in addition to the canonical NodeId represented in this Attribute. A Server shall persist the identifierType and identifier NodeId elements of a Node as well as the Namespace Uri which the namespaceIndex NodeId element references. A Server may change the namespaceIndex NodeId element of a Node with future Sessions and therefore a Client shall not assume the namespaceIndex will not change. The structure of the NodeId is defined in 8.2.