What is the function of the 'fifth wheel' (sellette d'attelage) on a heavy highway tractor unit?

1. A heavy-duty steel coupling plate mounted on the tractor chassis that locks onto the semi-trailer kingpin, supporting front trailer weight and allowing articulation
2. A spare tyre mounted behind the cab for emergencies
3. A steering wheel used by the passenger assistant
4. A fifth wheel mounted under the engine to measure ground clearance