Question:
When I lived in Chicago, they always referred to big fires on the news as a 4-11 alarm fire, or a 3-11-alarm fire, never 4 alarm or 3 alarm.

Does anyone know why or what this means?

Answer: Former 911 dispatcher here. How WE used it in San Mateo County, Calif., is it meant how many different groups of fire trucks (we called them alarms) were assigned to a fire. For example, a typical structure fire was first given three engines, a ladder truck and a battalion chief. Then, if it proved more than the intial assignment could handle, the battalion chief on scene could ask for a "second alarm" -- three more engines and another ladder truck. And so on, and so on.

Most I ever saw while I was working there was four alarms. There was a five-alarm not too long before I got there. I've heard of seven alarms in neighboring jurisdictions.