Ok, Well as I said I was confused by the title of the thread. But the meaning became clear to ME when I read the post.
Some info about me.. I grew up on a dairy farm, So I know about farm tractors, My mother was a tax expert (It turns out) so I know about the tax exemption.
MOST farmers have their own Fuel storage tank.. For this reason they do not fill their tractors up at the gas station. They fill 'em up at home.
Of course "Most" might not = All.. I have filled up a lawn mower at a gas station on occasion.
I have also driven a Semi Tractor (and a host of other vehicles).. So I know that a Semi is also called a "Tractor-Trailer" rig.