That means that the axle weight rating of the tow dolly is 3500, the dolly itself weighs 800, so the available payload is 2700 lbs. That itself is the weight of the axle of the vehicle that you plan to tow - say your car weighs 3900, and you plan to put the front axle on, and the front axle weight is 60% of the 3900 (all extremely typical numbers) then the front axle that is the load on the dolly will be 2340 lbs, less than the maximum rated load of 2700 so you are OK to go.
Brian
2005 F250 Supercab, Powerstroke, 5 speed automatic, 3.73 gears.
20 ft race car hauler, Lola T440 Formula Ford, NTM MK4 Sports Racer
1980 MCI MC-5C highway coach conversion
2004 Travelhawk 8' Truck Camper