If you want motion not to be detected in a certain area of the camera image, you can mask out that area. Masking is started by checking the ``Masking'' checkbox next to the camera image. Then, you can use the mouse to draw an area on the camera image, which will be covered by a mask grid. The left mouse button does masking, while right button erases it. The size of the brush can be adjusted using the slider.
When masking is done, uncheck the ``Masking'' checkbox. The visible mask will be hidden, but remains active.