The halt notice a session sends to the processes a send processor starts from Statifier.Send.Processor.perform/2, such as the timer that holds a delayed send (ADR-0069 decision 4: the processor owns the delay).

Spec 6.2 asks that a delayed send be discarded when its session terminates before the delay elapses. A session's status can be :done, :cancelled or :budget_exhausted while its process is still alive, so a timer cannot tell from the process alone, and asking the session with a call can time out while the session is busy. Instead, a processor hands the process it starts to watch/2 from inside perform/2, which a Statifier.Session runs in its own process, and the session sends that process

{:statifier_halted, session, reason}

when it halts, session being the session's pid and reason one of :done, :cancelled or :budget_exhausted. A process watched after the session has halted is sent the notice at once. The session never calls a watched process and never waits on it.

The session also keeps the watched process under the key the processor gave it, so the processor can take/1 the processes held under a key (to cancel them, say), and forgets each one when it ends.

Outside a Statifier.Session (a host that performs instructions in its own process), there is no session to send the notice: watch/2 keeps nothing and answers :not_a_session, and take/1 answers [].