Just a quick question, when it says max gross weight of 38 tons on the trailer does that mean max gross weight Inc unit? the reason I im asking this is because we have daf85s 4X2 plated at 38 tons and we have daf85s 6x2 plated at 44 tons, I dragged in 42ton max gross weight with a 6x2 unit plated at 44 tons and the trailer said 38 tons , was I legal ? 
No, it means the max gross weight of the trailer.
So yes, you were legal.
As Simon said you were perfectly legal as the max weight on the trailer plate is the weight of the trailer+load but not including the tractor unit, and so given an average 6x2 unit will weigh about 8tons your trailer isn’t ever going to gross over about 36tons for a 44ton total gross.
Paul