Registration
Per RFC 7641 2. The Observe Option:
When included in a GET request, the Observe Option extends the GET method so it does not only retrieve a current representation of the target resource, but also requests the server to add or remove an entry in the list of observers of the resource depending on the option value. The list entry consists of the client endpoint and the token specified by the client in the request. Possible values are:
0(register) adds the entry to the list, if not present;1(deregister) removes the entry from the list, if present.