Combined rating minus the loaded vehicle is the room left to tow.
A 15,000 pound GCWR, a 5,500 pound curb weight, 400 pounds of people, and 300 pounds of cargo leaves 8,800 pounds of GCWR room. A published tow rating of 10,000 pounds does not raise that. The lower number is 8,800 pounds. Payload is a 7,000 pound GVWR minus the 5,500 pound curb weight, which is 1,500 pounds. Payload left after people and cargo is 800 pounds. That 800 pounds is the truck load calculator result for what can still go in the vehicle, not on the trailer. Tongue weight usually counts against payload. This page does not add it unless you include it in the cargo field. These are the ratings you type. They are not a scale ticket and not a rating Dengaro publishes.