I explained this earlier. A Quint (Q) in the fire service is a hybrid fire Pumper or Engine (E). The NFPA National Fire Protection Association has a book NFPA 1901 Standard for Automotive Fire Apparatus has designations for various use fire equipment. E for Engine / Pumper, L - for Ladder - no water pump, hose,
T - Tower Ladder / Truck (is dependent upon area of the country what they call them. Tower has a bucket at the end of an elevated Ladder, or elevated platform.) Q is a Quint. It is a hybrid Ladder truck that carries hose, has it's own onboard water tank, fire pump, supply hose and an elevated Ladder. There are also equipment requirements for the truck to carry to be considered a Quint. So, Q being Quint, 74 being the apparatus identifier; usually the station number that it responds from. 25+ year Fire Service Vet.