In the SMART space, fhirUser is allowed to be a Person reference because a patient proxy may be viewing FHIR resources for multiple family members where RelatedPerson isn't sufficient. If userId is allowed to include Patient and RelatedPerson references, Person should also be an option. Related zulip discussion: https://chat.fhir.org/#narrow/stream/179170-smart/topic/Survey.3A.20.60fhirUser.60.20for.20patient.20proxies.3F

Existing Wording:

The id of the current user.
For example, if the user represents a FHIR resource on the given FHIR server, the resource type would be one of Practitioner, Patient, or RelatedPerson.
If the user was a Practitioner, this value would be Practitioner/123