Code 1 is no emergency. No lights, no siren, you drive there like a normal person. Code 2 is a semi life-threat. No lights, no siren, you drive there like a normal person. But like you're late for work. Code 3 is a life threat. Lights and sirens for a rapid response, and allows ignoring most traffic laws (But not risk the life of others). Code 4 is "all clear".