My Santorin has a big brass plate that says net tonnage is 15 and 1/4 and gross tonnage is 18+. But the spelling is tons.
I know it refers to capacity, but I don’t know if there is a difference in where it was measured.
In metric tonnes the Santorin is 10 tonnes.
I give up !
Incidentally my Part 1 registration has the boat length at 12.3 metres ( rather than the 14 that it is). It has saved me money several times when a marina has just asked for my registration certificate!