chrismol1 is correct, its the EMD code. If you ever monitor ADI (Buffalo Ambulance Dispatch), they dispatch it by EMD code to AMR Dispatch (# - Alpha - #), or Bravo, Charlie, Delta....
It denotes what type of response it is. Every system is set up differently by the Medical Director for the dispatch center. On some dispatches in the area, you'll hear "Medic Hot Response" or "Basic Cold". Some dispatches don't even say because you're getting a medic coming on every call.
If I recall correctly, ECHO in Amherst means, and don't quote me on this, any available firefighter/EMS provider with "equipment" can respond to assist, regardless which fire department, assuming they are nearby. Normally, ECHO's are cardiac arrest, choking or those oh sh** need help fast calls where time is really of the essence. Every agency does it differently
This is a decent break down of all the "call types" and what their meanings are
Priority Dispatch Codes