The OP does not provide his location, so I don't know what agency/etc he is looking at. In the case of the CHP (California), the input is the frequency of the cars, and the "frequency" is the frequency of the dispatcher. In most cases you have to listen to both in order to hear both sides of a conversation.
What is really needed is some sticky or Wiki article that we can point people to who ask this question. Unfortunately it is not as black and white as just ignoring it 100% of the time.