Scantling draft is the maximum draft at which the ship can withstand all the loads safely. In other words, it is the draft at which the strength of the ship is built.
It may or may not be equal the tropical draft (Maximum to a ship can load as per load lines).
Let us say a ship's maximum depth is 15 meters. Its tropical draft is 12 meters. Let us say based on the strength of the ship, we calculate to how much draft this ship can be loaded and still the stresses on the ship will be in safe zone. Let us say we find this draft as 13 meters.
Off course we cannot load upto 13 meters. This is because as per load line convention we can only load upto 12 meters. But if it is safe to load upto 13 meters draft, why we are allowed to load only upto 12 meters. This is because load line drafts are not based on the draft that we can load. Rather it is based on the minimum freeboard we need to have at all conditions.