From The RadioReference Wiki
Many notice that in the Radio Reference Database an Input Frequency is specified in addition to an output frequency. The Input Frequency refers to the frequency that mobiles and portables transmit on to access a Repeater. The Repeater then rebroadcasts everything on the input frequency to all radios listening on the Output frequency (which is the one in the Frequency column in the RR Database).
In almost all cases, you will want to program your scanner with the Output frequency listed, since this is the frequency that the Repeater is using to rebroadcast all communications on the input.
You don't typically program the input frequency in a scanner for receiving since you will only hear communications that are close to your receiving station - line of sight. A repeater with an antenna mounted high up will have much better line of sight to mobiles and portables transmitting on the repeater input frequency.
Repeaters use both an input and an output frequency. The input and output are separated by an Offset which in many bands is standardized.
Note: some scanner listeners will program and listen to just the input frequency to catch operations that are only occuring very local to the listener.
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