The Radio IDs is the actual ID number of the radio transmitting at the time you see it. Each radio (mobile, portable & dispatch console) has a unique ID number and that is what you are seeing. This is used for various reasons: such as when a "emergency" button is pressed, the dispatcher knows exactly which officer hit the button, or if a radio is stolen, it can be "bricked" or disabled; and the radios can be numbered according to what agency is using the system (City, County, State, EMS, Fire, Police, Sheriff, etc.)
The following link explains the numbering a little better for the Michigan MPSCS system to give you an idea how it is numbered according to zone:
http://wiki.radioreference.com/inde...fety_Communications_System_Radio_ID_Structure