Rear sights for ARs have a click adjustment feature. The value of each click varies depending on sight radius—the distance between the rear and front sight. Each click will equal about 0.5 to 0.75 inches at 100 yards.
Most rear AR sights have two apertures to choose from. Generally, the large aperture is used inside 200 yards and the small one for 300 yards and beyond. An offset between the positions of the holes makes this possible.