Is it the rear end axle, the rear end gears, the tranny, the shafts? Is it the combination of all that?

For example, if you put Rockwells under it, but keep the drive shafts, the 200, the stock motor and tranny- then is it a 2.5 ton? Likewise if you keep the stock axles but swap in a 2.5 ton drivetrain then is it that rating?

I was just curious.
-B.