https://www.hl7.org/fhir/identifier-registry.html lists Medicare Beneficiary Identifier http://hl7.org/fhir/sid/us-mbi with a Type of SB.

However, in an internal mapping I just reviewed, I'm seeing the use of MC.

Is one of these more right than the other? Are they both acceptable?

UPDATE:  I got confused by the fact that the identifier-registry points to the values at

https://www.hl7.org/fhir/v2/0203/index.html and that this valueset exposed all codes from the v2-0203 CodeSystem.

Instead, I should have looked at https://www.hl7.org/fhir/datatypes.html#Identifier which indicates that it has an "Extensible" binding to https://www.hl7.org/fhir/valueset-identifier-type.html and so the type values must come from that list if there is one that applies (and I'd agree that SB does apply for this).

What might have helped me is if the type values listed at https://www.hl7.org/fhir/identifier-registry.html actually linked to https://www.hl7.org/fhir/valueset-identifier-type.html instead of https://www.hl7.org/fhir/v2/0203/index.html but this is pretty minor, so feel free to close it.

Originally raised on Zulip at https://chat.fhir.org/#narrow/stream/179190-united-states/topic/Medicare.20beneficiary.20identifier.20identifier.20type