The most difficult problem is with mapping (in UML) a global element to the namespace in which it is declared. The global element may be declared in a different schema module, and possibly a different XML namespace, than the complexType or simpleType on which it is based. Some XML Schemas (e.g. ACORD) use one very large schema file in one namespace; global elements are quite straightforward here. Other schemas (e.g. OAGIS 8.0 and 8.1) use a very large number of schema modules to support reuse and abstraction, which greatly complicates mapping global elements to schema modules and namespaces. UML has not yet settled on a rule for determining schema document modularity and their namespace assignment.

My interest in NDR recommendations is more general than UBL. I'm looking for a set of industry best practices. I am still testing alternative approaches to support global element mapping to UML and expect to find a workable solution, but use of local elements (or restricted use of global elements in some situations) simplifies the mapping and will reduce long-term maintenance of UML models used for system integration.