I can only answer you, that if you send a message in “Trade Chat” channel, then that message will appear in Ironforge, Stormwind, Darnassus and Dalaran (If Alliance that is), as Trade Chat is overall major cities, and not only designated for one particular city alone.
You can NOT send a Zone-Wide message, unless you are in that particular zone, with your character.
I have no idea, about Layers though, if i remember correctly, you could see other layered players’ messages.
Yet another of this person’s “common knowledge” questions was what I meant, if not what I said. I find this very strange, as he has been playing for 8+ years, and been in the Classic forums since they opened or thereabout. I really am nonplussed by this.