It allows users to input the dimensions of each rectangle, as well as a label to indicate what type of rectangle it is (e.g. wall, window, door), and whether to add or subtract its area from the total area.
For example, a user could input the dimensions of a window as 1000mm x 1000mm, and specify that its area should be subtracted from the total. They could also input the dimensions of a wall section as 2000mm x 3000mm, and specify that its area should be added to the total. The calculator would then display the total area as the difference between the added and subtracted areas.