Motorola trunking systems are broken into cores called zones. This directly correlates to the RFSS, i.e. a RFSS of 1 means Zone 1, RFSS or 2 means Zone 2, etc. To simplify the layout, radio communication takes play at two controller levels. The site controllers control the chaos that is trunking while the zone controllers hold the user/talkgroup databases and create links between sites for specific talkgroups.
Logically it looks something like this. When a radio attempts to access a site, the site controller asks the zone controller if that unit ID is allowed (this is called registration). If yes, the radio will then attempt to ask for talkgroup resources to be provisioned at the site. Again, the site controller will ask the zone controller and if yes, the process of affiliation has been completed. On the backend, the zone controller keeps track of what talkgroups are in use at what sites. If a talk group is used at multiple sites, the zone controller will "route" the talkgroups between the sites. If that info needs to go beyond the zone controller to an adjacent zone...the zone controller simply forwards it to the other zone controllers.
There are several types of Cores.
K-Core, which is a conventional core.
L-Core, which is sometimes known as a Lite Core as it is really aimed for the smaller systems that don't need hundreds of sites or only have a small simulcast system.
M-Core, which is the full featured, multi-zone capable setup.
@KevinC one thing just came to mind, I believe there are some limitations of M1's if I am remembering correctly. More than 2500 users requires a system license be purchased (not upgrading to a M2 though). Just came to mind as a local system recently went through that.