Caravan wheel bearings comprise of two parts, the cup which is pressed into the hub and the cone. This is the part that contains the rollers of the caravan wheel bearing. Caravan and Trailer wheel bearings should always be replaced in a set, that is the cup and cone together.
In a bearing combination in a hub, the bearing on the inside of the hub flange (the side closest to the centre of the trailer) is called the inner bearing.
The bearing on the outside is called the outer bearing.
Parallel bearings are generally considered better for off road use as they have a higher load rating (1600 kg). With standard bearings, the inner bearing is larger than the outer, but with parallel bearings they're the same size.