The short answer
Permitted total mass decides the licence class, towing capacity decides what your vehicle is allowed to pull, and nose weight decides how much may bear down on the tow ball. All three have to be observed at the same time, and all three are printed in registration document part I.
Anybody towing for the first time reads up on exactly one number — and leaves out two more that are just as binding. They answer three different questions, which is also why they cannot be traded off against each other.
The three numbers and what they answer
- Permitted total mass (field F.1): what the vehicle may weigh loaded. The sum for towing vehicle and trailer gives you your licence class
- Towing capacity, braked and unbraked (fields O.1 and O.2): what your vehicle may pull. A question of engineering, not of licences
- Nose weight: what may bear down on the coupling. Printed in the registration document and again on the tow bar plate — the lower of the two applies
Why they do not substitute for one another
A car can have a high towing capacity and still form a combination that class B does not cover. Conversely a combination can be fine in licence terms and exceed the permitted towing capacity — in which case no entitlement is missing, but the journey is still not lawful. Both limits apply side by side, and the lower one always wins.
Nose weight is the underrated one
It sounds like a side condition and it governs how the outfit behaves. Too little nose weight — a trailer whose load sits too far back — is the most common cause of an outfit that starts to snake above a certain speed. Which is why heavy items are loaded over the axle and slightly forward, never at the tail.
Anybody standing at our counter with the papers points at the towing capacity and means the licence question. We then turn the page over and go through the three fields in order, and regularly it turns out that the planned combination fails on an entirely different number from the one that brought them in. That is why we now always ask for both registration documents, not just the one for the car.
The 100 km/h approval is none of these
The 100 km/h approval for trailers is a separate matter with its own conditions — tyre age, dampers, the weight ratio. It changes nothing about licence class, towing capacity or nose weight, and replaces none of them.
Which class follows from your arithmetic is in the 4,250 kg calculation, and the comparison of the two routes under BE or B96. Training and procedure are on the pages for class BE and B96.
What to take away
Three numbers, three different questions, and none of them replaces another. Check all three in the same sitting when you plan an outfit — otherwise one is settled and the journey is unlawful anyway.