Comment:

The Subscriber need not be mandatory as the element would redundant with the Coverage.identifier except where the Subscriber is a different party than the Beneficiary and is needed to costruct a memeberID, for example where the memberID is subscriber-dependent (123456-01, 123456-02, etc.)

To be clear the SubscriberID is not the primary identification for the Beneficiary or the coverage instacne to the payer, rather that is the Coverage.identifier.

Summary:

The Subscriber need not be mandatory as the element would redundant with the Coverage.identifier