The overlay is the amount the door is larger than the cabinet opening. The calculator applies a constant overlay all around the door. The door is larger than the opening by the same amount all around.

Setting this value to 0 creates a door with the exact dimensions of the opening.

A negative value creates an inset door.

On a double door the overlay is applied to the two doors as a unit. There is no gap between the doors. Both doors together are the same size and have the same overlay as a single door.