Each circuit has a unique set of rules. The basic differences between the classes are the weight and the number of engines.
Weight is probably the single most important factor in tractor pulling.
The more the tractor weights, the more traction it was, and the harder it can pull. It is a challenge to build a tractor to compete in the different classes. In the light classes we greatly exceed the safety requirements because we must be built to standards of the heavier classes. But, in the havier classes, we are at a significant disadvantage by being limited to a single naturally aspirated engine. In all of our classes, we are there to have fun, and we do indeed do that.