Bed capacity
Volume is cubic feet. Weight is payload. They are not the same limit.
Bed length, width, and depth tell you whether a sheet of plywood, a motorcycle, or a camper sits in the box. Width between the wheel wells is the number that matters for a flat load. Depth matters once you stack past the bedside.
A bed can look empty and still be overweight if the cargo is dense, or look full of light cargo and still be legal. The pounds come out of payload. The shape comes out of the box dimensions. This catalog prints bed length only when a towing guide names the box.