I have the exact same scope on my .243.

Your owners manuel explains how to focus the reticle and properly use the AO (adjustable objective)

The ocularlens (part closest to the eye) turns to adjust the reticle focus for your eye.

The purpose of the adjustable objective lens with the yardage settings, as glockman88 correctly stated, is make the image parallax free for the yardage in which the shot is to be taken. Typically 4X12 and higher magnifications range scopes must have an adjustable objective to correct parallax. Parallax occurs when the image and the reticle are not in the same focal plane and the aming point (crosshairs) move on the target when you move your head horizontally while viewing the image.

You will notice thatan image will come into better focus when the ocular lens is adjusted correctly for the distance being viewed. Also, the yardage marks may or may not be accurate, which is something you will need to pay attention to.

In general I leave mine on about 100-150yards when hunting open terrain and 100 yards or slightly less when hunting in the woods. If a long shot presents itself then I will adjust the AO as indicated, otherwise I leave it alone.