Rog,
Isn't the confusion over excessive MAM from this, where the MAM of the trailer exceeds 3500;

From above;

Trailers over 3500 kgs plated MAM weight come under different rules which is why all the trailers towed by B class vehicles , those of 3.5 tonnes and under, are not plated at more than 3500 kgs or 3.5 tonnes MAM