The others are onto it. Half, three-quarter, one-ton, it's an old parlance.
The last vestige of it was let loose in the late 70s. Up until that break, Ford 1/2-ton pickups were F-100s. I don't recall the year, but they suddenly were labeled F-150s. They (on paper anyway) upped the rating on them so that they could continue to burn unleaded gas. This was back when both leaded and unleaded were both still available.
I'll stop now that I'm showing my age, but the others, GM and Dodge did the same.