Depends on where that Knob is located there MattyT.
If it's at front of Scope, then it's a Parallax/Yardage adjustment, to fine tune the clarity according to distance. If you have Rifle zeroed in at 100 yds and you try shooting at 500yds, it will NOT compensate. Your Bullet will not hit Target, due to it's only zeroed for 100yds.
If Knob is where the Elevation Knob is, then once you Zero(then Knob is re-set back to 100yds according to Manual), then by turning to 200, 300, 500 yards, the Scope's Crosshair will compensate to that distance without you(the shooter) from having to re-zero each time. This only corresponds on what Ammo is used for Scope's tests, which will also be in Scope's Manual.
It could be a Side Focus/Distance knob also. You Zero your Rifle(100yds) the regular way with the E/W Knobs, and this Knob set to 100yds. When you look at objects/Targets at greater distances, you adjust this knob to where you get sharp picture. This will be your distance/yardage. If Scope has Markings/Dots under the Crosshair, you sight using these Marks. These are calibrated for that distance.