The ‘Mitres’ part of the dialog enables windows and doors to be placed at the intersection of walls, and to create a corresponding opening there. On the left you see a window where a mitre is allowed, and on the right where one is not allowed.
The ‘stop’ part of the dialog also changes the settings for the (invisible) 3D body which surrounds the window and is deducted from the 3D body of the wall. The results can not only be seen in the 2D views, but also in 3D. In the 3D view the window objects are set as invisible, so that the stop can be seen.