Patient+Discovery+LivingSubjectID

The Nationwide Health Information Network Testing team posed the following question regarding the use of LivingSubjectID in Pateint Discovery
[|JoeLamy] What is/are the accepted forms of leaving out the patient ID in a Patient Discovery request (i.e. using query only mode)?

Since LivingSubjectId itself is optional, omitting the entire element seems to be what the spec intended. But a question came up about passing LivingSubjectId with a root and leaving the extension empty.

The only clarification I could find is in the HL7v3 schema (II type), which says extension is optional. So the following would be legal in LivingSubjectId:

  

Which of these are allowed for a Nationwide Health Information Network PD request, and what would each imply?

Guidance Pending