Well.... in simple terms elevation and windage changes are made on the 'scope using one of the turrets. As you wind them around you will feel a click, and each clicks raises the POI (Point of impact) by a set amount, the problem is the clicks equate to an angular measurement. This means 1 click at 1000 yards is 10X more than 1 click at 100 yards.
'Scopes are usually set in either MOA, or minute of arc which is approximately 1 inch at 100 yards, or 2 inches at 200 through to 12 inches at 1200 yards.
The scopes are graduated in fractions of an MOA, usually 1/4MOA (1/4" at 100 yards or 4" at 100 yards etc. Some are also graduated in 1/8MOA and some in 1/5MOA (Unusual)
So if you are shooting at 600 yards and are 12" low you need to raise the elevation 2MOA.
Simple innit?
Milrads to confuse things are different. S&B's Mildrad 'scopes are graduated in 1/10th Milrad clicks which is around 1/3" (.344 to be precise) which is 1cm/100m or 10cm/1000m. So if you are shooting 1metre low at 1000m you need to go up 10 clicks. Confused yet? :?
So what do you go for? I would say milrad anyday, and a 1 turn turret, this means 1 turn of the turret will take you up 130 clicks or 13 Milrad on an S&B 'scope. This means you can never be confused or loose yourself with a turret that turns multiple times.
I try to think in metric, with wind in m/sec and I know at 100m a target moving at medium walking pace needs 1 click of lead, fast=2 clicks pretty well regardless of distance for my rifle taking time of flight into account, sounds odd but I shoot 'Huns heads' (A cutout German head target) at 600m both stationary and moving with three second exposures followed by 5 seconds down. Then they come up again in a different place for 3 secs. I have to be very quick to get on the target and know how much lead to give it hence milrads suits me.
Hope that helps!