For accuracy, the owners manual says:
"Do not tow a trailer at all during the first 500 miles
(805 km) the new vehicle is driven. The engine, axle or
other parts could be damaged."
and
"Then, during the first 500 miles (805 km) that a trailer is
towed, do not drive over 50 mph (80 km/h) and do not
make starts at full throttle. This helps the engine and
other parts of the vehicle wear in at the heavier loads."
So to the original question, I think you need to use some common sense as towing an empty flat bed 5x8 utility trailer weighing about 600 lbs is 'towing' and towing a 9,000 lbs travel trailer is 'towing' but clearly they are not the same. I think the point is to allow the truck, when under heavy load, a chance to break in, so they pick 500 miles (of initial towing) and 50mph.
My recommendation is 'take it easy on the truck' in your initial towing to let the engine, gearing, transmission, brakes, etc. have a chance to settle in under load - how long you do this will really depend on what you are towing and my interperation of the owners manual is it needs to be for a 'few hundred miles.
But, 500 miles of pure highway flat would basically never work the transmission gears or very many componets, compared to short trips with starts and stops etc.