Remember, trunking systems aren't really designed to accommodate non-affiliated "receive only" radio status. If the system doesn't simulcast a particular talkgroup at all sites, then unless your radio is affiliated with a given site that you can copy, you won't hear the traffic on that talkgroup. Just like your cell phone, the system tracks the status of all the radios affiliated at a given time and logs the info. The mobile/portable transmits to tell the system controller what its channel status is so that the controller knows "where" to poll or look for it if there's a call or a reason to check unit status. Radio status is available to the console operator for unit status/availability/busy/safety reasons. For instance, if Unit 101 calls dispatch and asks where Unit 102 is in the system, the operator can query that unit in the CAD or look at the CAD to see what that unit's channel status is as well as check his MDT status. Sorta like, "101, Unit 102 is currently on OPS 10 with 103 and 106, and the MDT and GPS indicate a traffic stop at XXX and YYY."
If I put my portable on a talkgroup that's used on the other end of the state, my radio and every scanner within receive range of the site I'm affiliated with will be able to hear trunked traffic on that talkgroup coming through the system from the other end of the state. As soon as I change talkgroups, the scanners won't hear the traffic on that talkgroup anymore because there are no longer any local radios affiliated with that talkgroup on the local site, and there's no longer a reason for the local site to be transmitting that talkgroup.
Maybe all that was clearer than mud.