That tire could be used on several different vehicles that might vary in weight. Your tire has an optimum footprint of so many square inches and requires a certain PSI to support the weight of the vehicle. So a lighter vehicle would require less tire pressure to support the vehicle with the optimum footprint. If you put say double the vehicle weight on that same pressure the tire would squat. So 45psi is the most psi that could go in the tire to support the truck and the maximum weight the tire could support at that psi is also printed on the tire. This is why when you go from a 35-12.50-17 BFG to a 35-13.50-17 whatever tire, you should lower the tire pressure because the footprint is much wider and the higher psi will make the truck ride harsh.
For stock tires, go by what is on the door. If you put those same tires on an F250, raise the tire pressure. If you put them on a Taco, they would require less pressure.