OK. I believe the way it works is that each member has a 4 digit ID. The radios are programmed with multiple channels. For instance, channel one is "open", just 160.280 pl 167.9 and no MDC. You'll hear everything the base transmits, and anything said close enough for your radio to pick up. Channel 2 would be 160.200, the "talk about" channel. Channel 3 might be "F group", for flatbush group, in which your radio is muted until and unless it hears the 3 digit "group number", in this case 430, after which it unmutes. So if the base had a call on Ocean Parkway and Avenue T, the base would tx a 430, opening that group, and then say "any units available Ocean Parkway and T, patient not feeling well" or whatever the call is.....
Channel 4 may be "private", in which the radio is muted until it hears that unit's ID. So if that member had his radio with the ID of 5252 or whatever, it would be silent unless the base transmitted an MDC 5252 to "open" his radio. This is useful when the member wants to get some sleep, for instance.
Obviously this is just an example.
Dovy