3.6.5.3 Error Responses
The server MUST send back a 400 response when the incoming XML document is not a valid http://schemas.microsoft.com/2006/09/sip/presence-subscribers document.
The server MUST send back a 400 if the publisher sends a setSubscriber request with a subscriber URI that cannot be found in SubscriberSet.
The server SHOULD send back a 403 Forbidden response if the From-URI and the To-URI are not identical<38>.
The server MAY send back a 413 response if the total size for context elements in SubscriberSet exceeds an enforced server limit.
The server SHOULD send back a 413 response if the number of entries in SubscriberSet exceeds an enforced server limit.