How is a semi-trailer coupled, and how is its weight carried?

1. It is coupled only by a rope, and it carries all of its weight on its own axles
2. It is a self-propelled vehicle that can also be towed
3. Part of it rests on the motor vehicle, and a substantial part of its weight and of the weight of its load is borne by the tractor
4. It is a centre-axle trailer that never transfers vertical load to the tractor