RE: Line of Sight - Line of Bore Qn.
It is just the difference in the height the scope is above the barrel. The only way to adjust it is having a lower or higher mounted scope. With see though bases that you can use the open sights with is a good example. The scope will be sitting much higher and the angle between the barrel and scope will be much more. This is also combined with the balistics of the load that you are shooting which creates the arced path of the bullet.