Could you define "extensions", what they're used for, why I would create any, and how they would be implemented?  Guess this is an undocumented feature -- or if it is documented, I haven't found it yet.  I'm under the impression that crew call-outs for V-speeds are supposed to be here?  I get the chatter associated with push-back, etc., but no "real" call-outs from the copilot.  Your assistance would be appreciated.