BDC is usually setup for ranges 100yards, 200yards, 300yards, 400yards, etc. And you have to go and find the power to set your scope too for those ranges. Where mildot is has dots that are spaced out. It is usually used to approximate distances by knowing various sizes of objects in conjunction with target turrets. But also can be used for drop, or wind or lead in a hurry where you don't have time to setup your drop with target turrets. But you need a mildot ballistic calculator to know the mil holdover. And its different for different guns, velocities, and even ballistic coefficients. For example with a 308 with .4 BC at 2700fps with gun sighted in at 100 yards, you want 3.4 mil holdover.