I describe it at the end of this section[1]  but it may not be not clear enough.

One each POST request, the client will receive the N most recent notifications that were emitted since its last POST request. Once the POST response is sent, the server will flush the notifications held for this client. He will not receive twice the same notification.
It is possible to miss notifications if the client does not poll often enough and there are more than N notifications emitted. This is a compromise to prevent exhausting server memory by having clients register a listener and never poll its notifications that'll fill up the memory.
Having a sensible value for N (e.g. 1024) should leave enough time for a client to poll regularly and not miss most notifications.

Does that answer your question?