A tractor unit is a characteristically heavy-duty towing engine that provides motive power for hauling a towed or trailered load. These fall into two categories: heavy and medium duty military and commercial rear-wheel drive "semi-tractors" used for hauling semi-trailers, and very heavy-duty typically off-road-capable, often 8x4 or 6Ă—6, military and commercial tractor units, including ballast tractors.