Manufacturers maximum towing capacity - ATM or GTM?

Yes

VSB1 and ADR 62/02 clearly state ATM.

When manufacturers quote Aggregate Trailer Mass it means the maximum mass the towing vehicle can tow (mass of trailer + load in trailer) which may be less than the Gross Trailer Mass which is the total mass of the trailer fully laden ( mass of trailer + maximum load the trailer can carry) which could exceed the tow vehicle capacity
The ATM less the mass of the trailer is the maximum load you can legally put in the trailer regardless of the carrying capacity of the trailer
Cheers Kev

And to complicate matters - if you have a 5th wheel towing attachement and trailer configuration, then it alters again from what the manufacturers quote.

Often the difference between what downweight your tow hitch is rated for when loaded at 10% of GTM Gross Trailer Mass, (eg 350 kilos downpressure for a 3500 kilos GTM rated braked tandem trailer, on 50mm ball, with load levelrs weight distribution hitch and breakaway electric or vaccum operated brakes) is LESS than the payload capacity in the tray of ute with the 5th wheel hitch located over the back axel.

So typically 5th wheelers can tow more than a normal hitch.

Also - if you do happen to have a vehicle with a manufactirers rated tow capacity greater than the usual 3500 kilos of the big 4wds Like for example a Dodge or Ford light Truck with 4250 kilo tow capacitys - you can then use 70mm tow balls and the aforementioned devices to tow in excess of the 3500 kilos

You can be assured the Road Traffic Authority weighs and measures police - who have the portable digital scales will know for sure what the regs are for most combinations and have the ability on the side of hte highway to stop you and weigh you & issue the approprate fines etc.

Probably one of those guys would be the best possible source of good/reliable advice in this regard.

Cheers