We added some clarifications in 1.05:
Every UANodeSet that defines DataTypes has associated schema files that define how to serialize the DataTypes using the UA Binary Encoding (5.2) and the UA XML Encoding (5.3). The XML schema file may be referenced by the UANodeSet if it includes Variable Values (F.8) for one or more of the DataTypes defined in the UANodeSet. The namespace URI assigned to the XML schema is set by the tool that generated it. The URI for the XML schema for OPC UA DataTypes is https://opcfoundation.org/UA/20...../Types.xsd.
IOW, the elements in the DefaultValue value always use a different URI than the UANodeSet schema URI.
Any prefix can be used but uax: is what the tooling currently uses.
Note that serializing Structures of DataTypes defined in non-UA namespaces would reference a different schema and require a different prefix assigned in the XML file.
The XML schema for a NodeSet is usually published as a separate file but it is also the value of a Property of the DataTypeDictionary Node for the XML Type System that is part of any 1.04 or earlier NodeSet.
In 1.05 or later a new XmlSchemaUri will be added to the UANodeSet element.