Echo Level ("E") is the most serious - literally dying right now (unconscious, not breathing, no heartbeat).
Alpha Level ("A") is the least serious (Twisted ankle with no other symptoms such as bleeding).
There is also an Omega Level, though it's unlikely these will result in fire dispatch. Calls in this category may or may not result in EMS response, rather consultation with a medical professional. For example, "my child swallowed a penny seems to be okay, but I'm a little worried".