My squad's Captain is also the OEM Assistant Coordinator, Twp EMS Coordinator, County EMS Coordinator, a NJ EMS Task Force Leader. He generally uses whatever he feels is appropriate for the occasion. His vehicle is marketed though for OEM/HazMat use. This is how he generally uses for his radio callsigns.
34-3-50: for general ems calls, MVAs, fire calls, mutual aid to other areas
34-50: Structure Fires, Larger MVAs/extrications/Water rescues
34-96: OEM/HazMat calls, Water Rescue calls,
96-50: County assignments as a coordinator
In my county it is laid out what every type of vehcile, unit, officer, individual will be with regards to radio designation. There is little room to move.
30-32: OEM Coordinators
50-54 EMS officers (Capt, 1st Lt, 2nd Lt, Sgt/engineer)
66: Fire Chief
67: Asst/Deputy Chief
68-70: FD OFficers (Capt, 1st Lt, 2nd Lt)
All vehicles for command are simply marked with their corresponding Agency and Radio Callsign denoted by our county system. Some Additional may say EMS Command, Incident Command, Command, Chief, Captain, Coordinator, Etc.