The zones are a grouping of talkgroups, generally that will have some sort of relation to each other.
In Washtenaw County, the template is somewhat standardized. Using the template I have for a fire department:
Zone A - Agency's discretion
Zone B - County Interoperability (InterOp and 81 Com)
Zone C - Department local-use talkgroups. (One per department.)
Zone D - Medical assignments (Hospitals, statewide landing zones, HVA)
Zone E - Statewide usages (MSP regional dispatch centers, landing zones, non-MPSCS system links)
Zone F - I-Call and I-Tac
Zone G - Statewide Event 1-15
Zone H - Statewide Event 16-30
Zone L - Livingston County talkgroups
Zone M - Monroe County talkgroups
Zone P - Police/Sheriff talkgroups
Zone UM - U of M analog system backup
Zone W - Washtenaw County analog system backup
Law enforcement agencies will also have a Zone I in their radios, which are Statewide Event talkgroups, which are limited to law enforcement usage.
The radios that are used by the county are able to scan 15 items. However, they must all be in the same trunking system, or they can be conventional. You can not mix and match, meaning you cannot scan the county's fire, police and EMS dispatches along with the I-Tacs. You can scan Washtenaw, Livingston and Monroe dispatches, as they are all on MPSCS. I have not heard about any of the newer radios being introduced by Kenwood and Tait, they may be able to do mixed-mode scanning, and might allow for larger scan lists.
Zones F, G, H & I (if applicable) will be the same, on all radios. (I am drawing a blank on Zone E being mandated, like the ones just listed.)
Hopefully that didn't confuse you further.