In the summary in section 2.4, I think you use:

• "subscriber" to mean "the client that creates the subscription" and "the subscribing client"
• "client" to mean "the server hosting the callback URL" and "system receiving notifications"
  Please standardize your terminology.

Existing Wording:

The client that creates the subscription MAY NOT be the same system as the server hosting the callback URL or connecting to the WSS URL (e.g., a federated authorization model could exist between these two systems). However, in FHIRcast, the Hub assumes that the same authorization and access rights apply to both the subscribing client and the system receiving notifications.

(Comment 10 - imported by: Lloyd McKenzie)