If you are referring to the column marked "input", this is only applicable when a repeater is used. Mobile units, and sometimes the dispatch, will transmit to the repeater on the input freq, and the repeater (usually located at a high elevation) will REPEAT the transmission on the Output, which is the the 1st column. Normally, you do not need to program the input. However, some folks like to listen to the input because they can very quickly determine if a mobile is close by (typically 3-8 miles). That way, they don't have to listen to all the traffic for the entire county.