If you are towing a heavy load, you are going to need to examine your GVWR (gross vehicle weight rating), GAWR (Gross Axle weight rating) and GCWR (Gross combined weight).
I think the point highbeam is trying to make is that the max trailer weight shown in you owner's manual is meaningless, as attempting to tow that can easily exceed the GCWR or the GAWR.
I don't tow much, but let's use my vehicle for an example.
It's a '99 F150 Super Cab 4.2L, 3.55 gears, automatic tranny.
The max trailer weight per the manual is 5,600 pounds. BUT... the GCWR is 10,000 pounds and the truck (empty) weighs 4,530. I weigh 200 pounds, probably have 70 pounds of misc. tools. The truck holds 25 gallons, or 150 pounds of fuel. With just me in the cab, we are talking 4,950 pounds, so anything over a 5,050 pound trailer is exceeding the gross combined weight limit. No way I can leagally tow 5,600 pounds unless I take the spare off, remove the tail gate, go on a diet and have no fuel in the tank.
My truck's max. gvwr is 6,000 pounds, 3,100 max front axle, 3,200 max rear. If I add passengers in the cab and some gear in the bed, the toungue weight of even a small trailer could push me over the GVWR or GAVR. This is the situation with many SUV's and light duty trucks.
With a heavy trailer, the math works against you in a hurry if you are carrying passengers or stuff in the back, so you really have to take the max trailer weight rating with a grain of salt.
I know, people overload all the time with no problems, but you could be in deep manure if you cause an accident.