What does the 3/4 ton refer to?

(Threadjack I know, but damn the curiosity is killing me)

 

1/2, 3/4, and 1 ton used to refer to the payload capacity of a truck, but now it's used more as a model or type of truck.
F150/Chevy 1500/Dodge 1500 = 1/2 ton
F250/etc 3/4 ton
F350/etc 1 ton

A fairly modern 3/4 ton truck can actually haul double the classification (or more), and 1 ton dual rear wheel trucks can haul 2 or 3 tons in the bed. The new axles and suspension components are pretty stout. The limits are now more about the size of the brakes.