GVWR, Payload, Towing Capacity or GCWR

You will have some amount of hitch or pin weight - Your rear axle, suspension and wheels have to have the capacity to carry this additional weight. The vehicle weight you are towing is added your actual loaded vehicle to give you a combined rate. Ideally, you will get actual weights for everything and adjust accordingly while others will approximate with worst case scenarios where everything is loaded to maximum to determine what they can tow. There some people that will use the information from the manufacturer on RAWR, GVWR and GCWR as an absolute limit and others will research the components installed in their trucks to find the weakest component that results in the rating and upgrade it to give them an actual higher rating even if it does not reflect so on paper.