I think this explains it well.
GLOSSARY OF TERMS
ATM (Aggregate Trailer Mass): The mass of the laden caravan when carrying the maximum load recommended by the manufacturer – including water, luggage, the lot. This includes both the mass resting on the wheels of the van, and the mass resting on the towbar of the tow vehicle. This figure must not exceed the rated towing capacity of the tow vehicle.
GTM (Gross Trailer Mass): Almost the same as the ATM, but always a lower figure. This is what rests on the rig's tyres, but does not include what rests on the towbar when coupled to a tow vehicle. Like the ATM, it includes the maximum carrying load recommended by the manufacturer. This figure must not exceed the rating for the axle group (wheels, tyres, suspension and axle) specified by the chassis manufacturer.
Tare: Effectively the "empty" weight, this is the mass of the van with all OEM equipment fitted, but with no luggage or personal effects on board, and with empty water tanks. This is what a van weighs when it leaves the production line. Like the ATM, but unlike the GTM, it combines the mass that rests on the tyres and the mass that rests on the towball.
Ball weight: The ball weight (or ball load) is the load (or force) exerted on the towbar of the tow vehicle. The ball load should always be eight to 15 per cent of the ATM. Vehicle specifications include a ball load figure, although it is not generally as easy to find as the rated towing capacity. A van should always be loaded with any heavy items near the axle, and not at the extreme ends where they will alter the ball load.
Load-carrying capacity: The total permitted load that may be legally carried in the van – your water, luggage, food and the like. Subtract the Tare from the ATM and you have the load-carrying capacity. Any options or accessories not included in the Tare will obviously reduce the load-carrying capacity available for luggage and personal effects.