Sometimes, you may need to deduct an area from a shape that you drew, for example to take out the surface of openings to calculate sidings.
To do this, right-click on the area that you drew and select "Subtract from area". Start drawing the area that needs to be removed, and when you are done, click on the "Stop" button (or press the "Escape" key on your keyboard).
The area you just drew will be deducted from the total area.