One more point of clarification - it's not actually REL that determines whether or not players can chat during deck building; it's whether or not decklists are being used.
Originally posted by MTR 2.7:
Players in individual Limited tournaments using decklists must refrain from communicating with, or revealing hidden information to, any players or spectators until after they hand in their decklists.
Arguably, events that justify the use of decklists - those with significant prizes - should be run as Competitive, not Regular REL. (At least, that's my 2c worth.) But if you have a Regular REL Sealed Deck event where you're requiring deck lists, then don't let players talk during deck building.