In my old department we went from straight unit numbers with no meaning to the new system of station/response. Engine 11 is first out engine from station 1, Engine 12 2nd out, Eng 21,31, etc. Rescue units same system. Ladder and Haz Mat are just L 1, L 2 and Haz Mat 1. Reserve units change to unit they replace. There will always

be an Eng 11,21,31,41,51,61 on the CAD