The slider is moved forward to set the sight for longer ranges. Read from the front edge of the slider. Numbers are meters- 100, 200, 300, etc. Yes, there REALLY are sight setting in the thousands- but this was intended for volley fire (you and 30 or so friends) shooting at an area target (artillery gun crew). Reduces having to aim in the air way above your target. When you do that, target is obscured by the barrel. Adjust the sights, and you hold on the target.
What we have here is... failure- to communicate.