The Cut-out tool is used to subtract things like windows or doors when measuring the sides of buildings using the area tool.
To cut-out an area, firstly, measure using the area tool on your plan then press the 'New Cut-out' option on the toolbar.
Now draw the area you want to cut-out over the area measurement. Double-click to finish your cut-out.
You will then see another area measurement in the Stage section, indicating that there was a subtraction in your area measurement.