Tractor is described as a machine designed for towing. I.e. farm tractor, caterpillar tractor, highway tractor.
See also:
"Tractor" is in the NAICS Code "333112"
(Lawn and Garden Tractor and Home Lawn and Garden Equipment ManufacturingMulchers, lawn and garden-type, manufacturing)
"Tractor" is in the UNSPSC Code "14111504"
Tractor feed paper
Ref: 124173/2006-09-22