DWT is a measure of the weight (or displacement) of everything that a ship is carrying. It is equal to the entire displacement (or weight) minus the weight of the ship itself. So it includes the cargo plus fuel, supplies, people, ballast, etc.

DWT is the measure by which ships are generally categorized by size.

DWT provides a rough approximation of the weight of cargo (DWCC) that a ship can carry. This is usually around 95% of DWT.