Let me explain what he means: On a normal scope with mil dots, depending on the range, you have to hold at that mil dot to account for bullet drop. I know you all understand this point.
For Example: If a character is 200m away at the same elevation as you, then you would use the 200m mil dot to aim at him. This would account for the bullet drop and should hit right where you are aiming.
Now, lets say he was 200m away but at a lower elevation. the 200m mil dot would not be accurate because it is not accounting for the elevation difference. You would have to move the mill dot LOWER on target, even though the DISTANCE to you is 200m. You would have to adjust slightly to account for the difference in elevation.
What the developer is saying, is that the mil dots will move dynamically depending on the elevation of your target. So, YES... if the target is 200m away but at a higher elevation, the mil dots will adjust to account for the elevation. So you don't have to approximate "OK he is 200m away but at a higher elevation.. I will hold over the 200m mil but aim a little bit higher to account for the higher elevation"
The scope mil dots make this adjustment for you.