With regards to this topic, I looked at my 1960 Ford Truck service manual. From this:
F-100 1/2 ton
F-250 3/4 ton & 1/2 ton
F-350 1 ton & 3/4 ton
F-500 1-1/2 ton & 1 ton
F-600 2 ton, 2-1/2 ton, & 1-1/2 ton
F-700 & F-750 3 ton, 2-1/2 ton, 2 ton, & 1-3/4 ton
F-800 & F-850 3-1/2 ton, 3 ton, & 2 ton
F-950 5 ton, 4 ton, 3-1/2 ton, & 2 ton
F-1000 5 ton & 3 ton
F-1100 5 ton & 3 ton
So, you could get a 2 ton truck in several flavors, F-600, F-700, F-750, F-800, F-850, and F-950.
I usually don't stop in here but the title of this thread caught my eye. While all are good points of views, I saw no mention of where I believe the term came from.
I'm sure it is a military term used back in the 30s & 40s, WWI & WW2. I was born just after WW2 and heard those terms as a boy growing up. That was before any of the modern F series numbers were available.
I believe the terms were used both for trucks and trailers used by the Army.
As it pertains to this particular forum, it was a carry over from the WWII build days. Prior to WWII, there really wasn't a designation, or at least not a uniform one. After WWII, the terms were carried through to the civilian market after manufacturers stopped producing military hardware. It was a very "vague" basic designation for a particular vehicle, and it was "supposedly" the amount of payload a truck could haul when fully loaded with fuel, passengers, and all its accessories. We all know that every rig should have at least a 1/2 added to them. The reason for the various ratings in the same sized rigs (F-700, F-750 etc.) is mostly because of the rather large number of available spring options, wheelbase options (a shorter wheelbase will haul a larger payload due to centering rules), and tires, etc., etc. It actually means nothing anymore, and hasn't since the 50's, shortly after it took hold, since every model could be upgraded with heavier duty suspensions, etc., etc.