(Before you go on about how people have had topics about MA before, I'd like to present what my area does and see how other people do it similarly or differently)
Okay. In the national capitol area, back in 2007 there was some large meeting between all the major counties and their respective FD's. All units in the NCA now have a 3-number unit designator, with the first number denoting the county, based on this:
Units with a 0 or only 2 numbers come from Washington D.C. itself
100 Series: Arlington County
200 Series: City of Alexandria
300 Series: Washington Metropolitan Airports Authority
400 Series: Fairfax County
500 Series: Prince William County
600 Series: Loudoun County
700 Series: Montgomery County
800 Series: Prince George's County
The second and third numbers denote the station the piece comes from. For example, I live in Fairfax County, and my local station is 31. The engine from Station 31 in Fairfax County is Engine 431 or E431, the medic is Medic 431 or M431, and so on. Sometimes the stations have multiple units with the same designator, those stations are the volunteer ones, which are also staffed by career personnel. The second piece would be Unit Designator-B(ravo). The other exception is when a station has two ambulances or more ambulances. The first would be ambulance UD, the second Ambulance UD-B, and if the ambulance was only ever staffed by volunteers with EMT-B training, it would be Ambulance UD-E Closest unit responds, and additional units in the original box or subsequent alarms also could be MA.
Thats how we do it here, what about other people?