Trailer tow ratings are generally calculated by the vehicle manufacturers as GCWR (gross combined weight rating - the maximum that a vehicle and whatever it's towing can weigh in total) minus the vehicle's curb weight. The problem is that the figure the manufacturer uses for the vehicle's curb weight is usually for a base vehicle with only a 150 lb driver.
This means that any difference between the manufacturer's published curb weight of the tow vehicle and the actual laden curb weight of that vehicle (i.e., the vehicle's weight with options and accessories, driver and passengers, cargo, etc.) must be subtracted from the trailer tow rating. That's why the trailer tow rating is generally considered to be a "fluff" marketing number that the manufacturer tries to set as high as possible to get bragging rights in their advertising.
To answer your question directly, then, yes, whatever weight you add to the tow vehicle has to be subtracted from the trailer tow rating.
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