In many parts of the country, the Alpha, Bravo, Charlie, Delta, and sometimes Echo designators are used by the dispatching agaency to designate a level of response.
Alpha is usually respond with traffic - no lights or siren.
Bravo is the same, but slightly higher urgency (no bathroom breaks enroute!)
Charlie is lights and siren, but lowest level of emergency
Delta is very high priority (cardiac arrest, vehicle entrapment, major trauma, etc.)
Echo is kind of a local option. Can be MCI, etc.
CT-EMT-P-CC, ERT, Flight Paramedic
That actually is it. Each service can adjust the levels for different responses. I am a dispatcher that uses this. It is from the National Academies Of Emergency Dispatch. It is an EMD system, that in my case, is for Medical calls. The NAED also has fire and police EMD as well. For instance ours is a follows,
Alpha: usually a bls call. may be with or with out lights and sirens. No FD response
Bravo: Same as above but certain calls will code with fire depending on the nature of call.
Charlie: ALS again depending on the nature and how the questions are answered by the caller.
Delta: ALS, higher response level with FD chest pain diff breathing priority symptoms
Echo: Is usually the repsonse for cardiac arrest. An echo in the program means, say if you were a lone dispatcher and had an echo response, you would put the caller on hold, ( yes you can put 911 on hold) dispatch your ambulance FD or whatever, then go back and finish the set of questions.
There is another level that can be used , I know Los Angelas uses it called Omega. What that is say a pt calls 911 for the flu with no priority symptoms, or even a maturity pt waters not broken no real contractions, it can refer them to their local doctors, or to a different number, or some have it set with a local taxi service to go pick up the pt.