These are the refusals of the writing direction, where the reading direction's are TranscriptFailure. A person who typed something and saw nothing happen is owed which of these it was: a session that is no longer there is theirs to act on — they start another — where a session that could not be started is not.
A session is a running process, and send writes to one this host started — so a session someone's terminal is running reads perfectly well through this contract and cannot be talked to at all. This is also what a page is told when the process it was talking to has ended.