Giorgio CANGIOLI raised the following comment:
Please clarify how receiver is supposed to univocally determine which kind of identifier have to deal with and how is supposed to be used in a multi- identification domain environment.
For example as Italian Citizen I have a Fiscal Code that identifies me (something like CNGGRG77E12G999T ) how receiver may understand that this code is referred to the Italian Fiscal Code and not to another identifier, or even to a Spanish Identifiers that have a very similar structure. Who guarantees that this code represent me and not another Spanish guy.
The assigning Authority might not be enough for this purpose since it may happen that the same authority issues more identifiers.
Uniqueness of identifiers is critical for interoperability.
I suggest to add an optional field to record the type of identifier used, but above all something allowing to univocally and universally identify the identification space that identifier is referring to.