For each system you create, you must have a separate set of talkgroups (which means you'd want a separate scanlist for those talkgroups).
If you create one system as a multisite system [and of course add the control channels for all the sites within that system that you want to monitor], that system will use one scanlist.
EX: I like to listen to the Ohio MARCS statewide system - five separate sites in particular.
1. I program one system and call it MARCS EAST.
2. In that system I enter the control channels for the five individual MARCS sites that I want to scan
3. I then add all the talkgroups I want to monitor to a scanlist (such as ScanList 2)
All of my talkgroups in ScanList 2 are set to be associated with the MARCS EAST system that i created above
I set the MARCS EAST system to "Multisite Stationary". That way, it scans every site for activity periodically.
I do NOT have it set to check all CCs in a single pass. The reason is that I have other scanlists (for another trunked system and a conventional scanlist) active at the same time. I don't want it to take 10 seconds (plus/minus) to scan ScanList 2 simply because I have the control channels for five sites plugged in. I want it to scan Scanlist 1, one active control channel in MARCS EAST in ScanList 2, then move on to ScanList 3. Then it come back around and repeats - and checks a different control channel (I assume) in Scanlist 2 each consecutive time.
If I DO set it to check all CC's in a single pass, then it is guaranteed to spend a large number of seconds on scanlist 2 during each complete scan of all my active scanlists, and I then miss too much traffic on my other scanlists.
You can have one system, programmed as multisite, using one set of talkgroups in a single ScanList.
Conversely, if you program a separate unique system, it has to have its own set of talkgroups. Those talkgroups could also be in the same scanlist as another system if you wanted. But this is very efficient if your intention is to monitored a wide area networked system that shares the same talkgroups. If this is what you are wanting to do, you wnat one set of talkgroups and you want to program a single system with the control channels of all of the sites within that wide area trunked system that you want to cover.