Focus adjusts the scope to the individual shooter's eye... this is a one time adjustment. (For geezers and wearers of prescription lenses this could be a multiple procedure event if new prescription arrive.) The purpose of this adjustment is to adjust the scope so that the image of the reticle (crosshairs) is properly aligned with the back of the shooters eye.

Parallax adjusts the target to the reticle (crosshair). The target image and the reticle must be on the same focal plane within the scope or there is room for apparent movement of the reticle on the target when the shooter'e eye moves (or is positioned differently at the scope).