The front aiming mask is what the athlete is physically aiming at. It has a circle cut out to represent the aiming bull. There is also a black square printed on it. This square is known as the alignment shape and is used by the camera to determine the angle of the aiming mask in comparison to the camera.
The back aiming mask is behind the front aiming mask and the target paper roll. It serves two purposes. First, to reduce the amount the target paper roll deforms when a shot strikes it. Second, to help prevent lead particles from bouncing back from the backstop.