The normative contract for the OPC UA XML encoded Messages is an XML Schema. This file defines the structure of all types and Messages. This schema captures normative names for types and their fields as well the order the fields appear when encoded. The data type of each field is also captured.
The normative contract for Message sent via the SOAP/HTTP TransportProtocol is a WSDL that includes XML Schema for the OPC UA XML encoded Messages. It also defines the port types for OPC UA Servers and DiscoveryServers.
Links to the WSDL and XML Schema files can be found in Annex D.