From my humble experience with tonnage calculations when designing and registering fishing vessels I believe the tonnage is linked to the volume of the boat, not its weight and is referring to ancient cargoes of barils or ‘tonneau’ in french.
I recommend that you ask amel as they can easily give you this from the linesplan or you need to spend some time out of the water with a theodolite and pick up the hull lines to calculate the volume. You then use the class formulae to get the tonnage value required by your specific country of registry.
It is interesting that it is required for Sailing boat of this size, usually in Europe it is for commercial vessels above 24m, hence many fishermen asking for 23.95m boats!
Don't get confused with Tonnes (metric) and Tons (imperial). Tonne is an alternate spelling used to describe a metric ton. It is almost never used in American English, but it is widely used outside of the United States. Again, a tonne, also known as a metric ton, is a unit of measurement equal to 1,000 kilograms. A tonne is larger than a U.S. ton.
Barry & Robyn
Tradewinds III SM # 171