The close focus adaptor is a very slight magnifying lens, that alters the focal length of the scope in a very minor way.
What happens is, with the scope wheel set on infinity, the plane of sharp focus will be at 50 feet away. Consequently if you keep the CFA in position you can't focus on targets more than 50 feet away. So you need a means of placing it into position for short range targets and taking it out of use for anything over 50 feet.
Turning the wheel to closer distances will bring nearer targets into sharp focus. Usually you can get down to 8 or 9 yards.
Most shooters place the CFA in the lid of a Butler Creek. This needs some precision as it is essential the CFA is perpendicular to the optical axis, otherwise it will introduce some shift in the POI.