Hi 602
To answer your first question, NO, the ULW of the car is just the car with no trailer attached. The nose weight of the trailer forms part of the phyical load the car has to carry.
Yes scientifically Mass and Weight are different. All objects have mass, and it doesn't matter where you take them, their mass remains the same, but depending on where you are the weight it exhibits may be different, so to compliment your description a 1Kg bag of sugar weighs 1Kg here on earth but it will only weigh approx 0.166Kg on the moon.
What causes the difference is the relative effect of the force of gravity. The moons gravity is about 1/6 of that on Earth, so it only attracts items with 1/6 of the gravitational force here on earth.
However there are years of traditions inbred into us that accepts that here on Earth and for all practical purposes when describing caravans a mass of 1Kg produces a weight of 1Kg and similarly for the imperial units also. The practical differences in weights due to attainable altitude are also small enough to be insignificant for caravanner's.
Unfortunately, the regulations and laws concerning transportation and how they're described have evolved over many years, and in different places, which means the same feature of a car or caravan may have different names or units and therefore acronyms. - so confusion can easily break out
But worse than that is the detail of what feature is being measured or described. There are some criteria we like to use that have no specific definition, and in some cases different manufactures may use the same name but mean something quite different. Kerbweight is a prime example of where there is no universal agreement on what is or is not included in the "kerbweight" AND its similar but not identical to "Mass In Running Order" (MIRO oor MRO) and again similar but different to "Unladen Weight" (UL or ULW)
Where towing is concerned and especially when operating very close to allowable limits, you have to be very careful to understand or take advice about the safe and legal issues. The devil is in the detail. Even then its not always clear cut, the advice offered by the PC web pages associated with this forum have some errors, but even the Govt web sites have some ambiguous information and some of it is can be interpreted contrary to the acts or parliament or regulations.
Its a minefield, but with some patients and steady progress it is possible to get to the crux of the issues.