...
The NEXUSe2e developers interpreted this to mean that the scope of any CPAId's uniqueness may be in the context of two trading partners, as long as the parties mutually agree. Developers of one or more other messaging systems require globally unique CPAId values, regardless of a mutually agreed more narrow namespace. For some connectivity scenarios, these two implementations are incompatible when CPAId values are the same across two or more trading partner pairs.
...