I think this would be better at the end of the section. It feels out of place to me, since the next part of this section provides a fairly good general overview of REST-Hooks, whereas this is a fairly specific detail of the mechanics.

Existing Wording:

When a Subscription is created for a REST Hook channel type, the server SHALL set initial status to requested, pending verification of the nominated endpoint URL\. After a successful handshake notification has been sent and accepted, the server SHALL update the status to active\. Any errors in the initial handshake SHALL result in the status being changed to error\.

(Comment 33 - imported by: Gino Canessa)