What is the "T" wire for on a thermostat?

I just purchased a Honeywell programmable thermostat to replace my old Trane thermostat. I have a heat pump system with auxillary mode. Installation is fairly straightforward as outlined in the manual but afterwards I am left with a wire that was hooked to the "T" screw and is now unhooked. I have two possible places to put it, either in "L" or "RC" or I could just tape it off with electrical tape if it serves no use. Ultimately the key is knowing what it does. Of course I tried Honeywell support but they are closed today and I would really like to get this project finished since I am so close to being done. Any help would be appreciated.