The number is the "weight class" of the vehicle. The weight class chart from TM9-2320-361-10-1 actually lists three different numbers for an M32A2, empty 6, loaded cross country (5,000 LBS) 8, loaded highway 11. This is based on the weight of the vehicle in metric tons (2204.6 LBS) then rounded. take an M35A2 at CC weight (13,530 truck+ 5,000 cargo) 18530 / 2204.6=6.13 T rounded 6, take the same truck at highway 23530 / 2204.6= 10.67 T rounded to 11. This system was developed to cross bridges safely, if the number on the vehicle is the same or lower than the number on the bridge it's ok to cross, if it's higher you might be swimming! in europe almost all bridges are marked, the marking is the same as the plates on the vehicles, a yellow circle with a number, not as common in the states except on military posts.