What it is asking you is to select the control channel for the system you are monitoring.
One of two thing are happening here:
1) you are seeing a 'voice channel' for a trunk system. That is why it disappears when the transmission is over.
or
2) you are seeing a conventional channel using P25.
If you are trying to monitor the MARCS system, only enter the following freqs in the system you create:
867.90000a 868.27500c
These are the control freqs for the tower in Mahoning.
Always clue us in about your location and what you are trying to monitor. This helps alot when you ask us for help.
Happy scanning

Danny