E is engine, AT is Aerial Tower (Truck company, or "hook and ladder), T is truck (see below), RS is rescue squad, A is ambulance, BC is Battalion Chief, C is Chief, SA is the safety officer, DC is probably Duty Chief, M is medic unit, RID is Rapid Intervention Dispatch, EMS 703 is the EMS (Emergency Medical Services) duty officer, FM is Fire Marshal. Aha, I just figured out a couple more... FCE231 is Frederick County Engine 231 and FCAT23 i sFrederick County Aerial Tower 23. For any newbees, an Engine is a pumper, a truck or tower is a "hook and ladder". The difference between a truck and a tower is that a tower has a platform at the top... a ladder truck just has a ladder that sticks up in the air. The Aerial Towers are usually just called Tower rather than Aerial Tower on the radio.
Hope that helps.
You asked about a "service engine" and a "service truck"... I think what you must be hearing is a little dispatcher shorthand, of a unit returning to service after a call, and the dispatcher saying, "Service Truck 127, 909"; ie "you are back in service, Truck 127, the time is 9:09. Could that be it?
AFRA goes this way... FR is First Responder. I think the first A is Advanced Life Support and the last one is Apparatus. What it is is a piece that has a paramedic and Advanced Live Support (ALS) equipment. I don't know exactly what the staffing and equipment is, but you get the idea... has a paramedic (or maybe more) and stuff like a defibrillator, etc.
DNR is do not resusciate, and there are two levels of that, A & B. I forget what the diff is and never quite understood it myself, but it basically means no advanced life support or CPR or heroics, just keeping the patient comfortable.
Check the Maryland Institute for Emergency Medical Systems Services website, I think there's a good technical definition of it there.
Maryland Institute for Emergency Medical Services Systems
If you aren't familiar with the local apparatus numbering system, give a holler and I'll help with that.