When you use a scope it defaults to aiming at targets within 100m, so the central marker of the crosshair is aiming directly at the target within 100m.
When you push Page Up, it adjusts to 200m, 300m or higher so that in theory when you have the mark on your target the bullet will usually hit that spot within the specified distance band (within each white square from you on the map)
I haven't put copious amounts of time into testing the bullet drop of gun & scope combinations, I mostly use the SKS with a 4x.
With that gun, I'd say anything sub 300m isn't worth adjusting (if a target is within 3 small white squares of the map from you); from my experience that's the sweet spot for when to start adjusting the 4x scope.
Christ, no wonder I suck tennis balls at sniping. Was trying to hit a guy last night with the AKM + 4x, he was quite far away but I emptied a full mag on him and didnt scratch him.
He knew is at well because he just stood there....
