UserMessage

A turn on the person's side of the conversation, and the channel that carried it when a person did not type it.

An application composes turns of its own — a prompt reacting to an event, an opening instruction — and the session records them where a person's words go. channel is the only thing that tells the two apart, so it is kept: absent means somebody typed this, and a name means the application said so. An adapter reports the channel a sender declared; it does not deduce one, so an application that declares nothing is taken at its word.