I only mention the following background information because I don't know to what extent you already know these things. So, please indulge me, just trying to be as thorough and helpful as I can.
They dispatch all the Fire/EMS out of one central location, including St. Charles County PD/Sheriff and the Department of Corrections. The rest of the police agencies in St. Charles County are dispatched independently via S.L.A.T.E.R. These agencies all have their own talk groups within S.L.A.T.E.R., whether they're Fire/EMS or Law Enforcement.
I think the terminology Uniden uses is confusing because, in my mind, what Uniden calls "Systems" I see as "Departments," and what Uniden calls "Departments" I see as "Systems."
Since my focus isn't Fire/EMS but Law Enforcement, I just have my St. Charles County Fire/EMS set up so that all the talk groups for Fire.EMS are under one favorites list, and I just listen for the address of the Fire/EMS call to know which agency is responding, and where the event is occurring.
If you're wanting to isolate the agencies by Fire/EMS agency, I would recommend setting each agency up independently from one another, each as independent favorites lists, each with their own assigned talk groups, giving them each their own Favorites List Name, so you can turn them on and off by agency favorites list "name." Then, you don't have to program systems or departments. Once you set things up that way, then all you have to do to use Favorites Lists is what you're used to doing, just turn on and off the independent favorites lists by Fire/EMS agency name.
There is probably a way to separate out the talk groups using what Uniden calls "Systems" and "Departments" to use quick keys under one Favorites List, but I think that would be cumbersome anyway, even if you were able to map it out, trying to remember which quick key is which agency when you want to turn on and off certain Fire/EMS agencies.
If I can't figure out this seemingly simple Department program, Quick Keys are beyond my ability.