The R4 Topic-Based Subscription Server Capability Statement describes the expected capabilities of a FHIR R4 server supporting R4 Topic-Based Subscription Server Implementation Guide and discusses the topic in narrative here.

The Exchange Capability Statement should have a a SHALL for Subscription for read and write, a SHALL for the $status operation on Subscription, a SHALL for declaring at least one channel type, and a SHALL of a payload type. As of now the description is only handled in the narrative.