Sorry Polymo, I need to correct a few statements of mine.
Local = the piece of the map you are currently in (generaly determined by zoneinfo).
Zone = the current map. (as soon as you teleport to a different map you get in the Zone chat that you are teleporting to a different map and if the new zone is pvp enabled, after you teleport you see there a counter  from 5 to 0 for enabling the pvp mode.)
As for the Trade chat it was Zone based.
p.s.: I just rechecked with JW and agrees.