Q stands for Quint ie: 5 abilities has a Pump, Carries it's own supply hose, has an elevated ladder/waterway, carries it's own ground ladders, and has it's own onboard water tank.
It's a combination Ladder truck / Fire Pumper. The National Fire Protection Association (NFPA) has different classifications for apparatus and what they need to fulfill certain duties. Not saying that the Q74 doesn' have a double meaning.