Channel

How a UserInput reached the session. Open vocabulary: Custom carries any channel name we have not given a dedicated case, so the enum is never closed against a new delivery path.

This is a declaration the sender makes, never something deduced: the vendor records how a turn was transported, not who composed it, so nothing can check this against the session. Interactive is the default, which means the fallback is "a person typed this" — a caller that composes a turn and forgets to name its channel gets the reading a person's words get. A reader of this field must therefore treat it as what the sender said, and never as an authenticated signal about who spoke.