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
From what I understand, an Echo level response means any member who has BLS training or above is to respond directly to the scene to immediately administer aid instead of meeting their crew at the hall if it would be quicker to do so. Most often ECHO is used for sudden cardiac arrest or a completely blocked airway when every second counts. Typically a rescue will roll first to an ems call with crew. An Echo response allows any truck with a driver to roll. For example if someone at the hall is assigned to drive an engine but not the rescue they take the engine, instead of having to wait for a rescue driver to show up.