DWT or called Deadweight tonnage is the total weight that can be carried by a ship including cargo, fuel, clean water, ballast water, equipment, and ship equipment.
There are also those who interpret DWT as a measure of how heavy a ship is to be able to be loaded (max load) to the summer draft at the Plimsoll mark on the water line with a specific gravity of 1.025 (the water where the kpl floats) so everything that can be loaded: the weight of the kpl crew, supplies, fresh water, fuel, ballast water too if any. The total weight of all cargo on board is called DWT. But it does not include the weight of the ship.
So, what is LDT?
LDT or Light Displacement Tonnage is the physical weight of the ship and is the measurement used when ships are sold as scrap metal.
Light Displacement is often also referred to as the displacement of the ship when the ship is empty and only contains the standard equipment required (if there is fuel and water only for starting the engine and water for filling the boiler).
Weight when the ship is empty = average draft when the ship is at sea.
The draft on the ship at that time (average draft) is called light draft.