The required nameUserAttribute attribute is the name of user-defined attribute type. The structure of the user-defined attribute can be identified by this name.
Source
<xs:element name="AttributeUser" type="AttributeUserType" substitutionGroup="Attribute"><xs:annotation><xs:documentation>The global AttributeUser element is a user-defined entity attribute which contains a binary array or any user-defined structured XML data.</xs:documentation></xs:annotation></xs:element>