All the MTAPD radio ID's are based on their CAD designation, with the only exceptions being a District commander say "2 C/O" which would be the captain/inspector in District 2. I've never heard any of the command staff use their radios in over 25 years, they use cell phones, but still have radios in their cars.
The MTAPD has nine Districts;
District 1 - Suffolk County
District 2 - Nassau County
District 3 - Queens and Brooklyn
District 4 - Penn Station
District 5 - Grand Central and 125th Street
District 6 - Bronx, and Westchester County, one exception to the rule, District 6 also covers the Bronx, using a District 5 ID.
District 7 - Putnam, Orange, Rockland and Dutchess counties, and another exception, they will also do northern Westchester.
District 8 - All of Connecticut
District 9 - Staten Island
So units will have the district number followed by their post/sector as their CAD radio ID.
A few more exceptions;
ESU - Emergency Services
Highway - Traffic Enforcement
K-9 - canine handler and canine
DD - Detective(s)
There are also three mobile communication vehicles, they are FC-1 MVC 1 and MVC 2.
Hope this helps a little.