I just bought a BC125AT and it is the first scanner I have owned that can do this. Think of it this way. The county you live in uses a certain frequency for their fire dispatch (I will use mine as an example...46.20 MHz). Now say that a neighboring county uses that same frequency for THEIR fire dispatch (or for something else like their fire ground channel or fire police), but you don't care to listen to them. If your county uses a tone, you can program that in and you will ONLY hear your county's fire dispatch...not the neighboring county. Hope that helps.
My Scanners: Uniden Bearcat BC355N, Uniden Bearcat BC125AT, RadioShack PRO-2014, Bearcat 210XL