What does the numbering system for trucks stand for?

what does 1500, 2500, 3500 stand for?What does the numbering system for trucks stand for?
They are used in marketing to denote half, three-quarter, and one ton rated pickups. They *used* to be roughly related to their payload capabilities, but modern pickups have much higher payload capacities now....it was always a marketing term, but even more so now...What does the numbering system for trucks stand for?
1500 = 1/2 ton


2500 = 3/4 ton


3500 = 1 ton





f150


250


350





E150


E250





And so on..................
Hauling capabilities.... 1/2 - 3/4 - and 1 ton.