Although I've contributed next to nothing to UBL, I do seem to recall a
discussion around this that is easily understood in the form of an
example (totally contrived) that would violate the rule:
"The CustomerID element may have the optional attribute customerIDType.
If that optional attribute is missing, then the customer ID is a DUNS
number."
(Yes, I'm aware of W3C XML Schema support for defaults, but that's
beside the point and probably not relevant at all given that the rule is
not stated in the context of XML Schemas.)
Regards,
Jim Wilson