CDS - custom dial system, doesn't matter what reticle you have, both FFP (first focal plane) or SFP (second focal plane) would work with CDS.
as all dials, CDS moves the whole reticle up/down. the duplex center is where the dial is set at. Leupold uses your ballistic information to generate a set of ballistic data, and it gets printed on to the CDS, on CDS there are distances. you use your range finder to find the distance between you and the target, and dial the distance on the CDS, and shoot. it's not most accurate, but great for hunting. if you do plan to shoot mid-range, you will need to account for wind drift. the CDS doesn't account the wind drift for you.
the difference between FFP and SFP: when you move through the zoom range, FFP reticle changes size (in high magnification reticle is bigger, but the center of the reticle stays at the same place in all magnification setting), size of SFP reticle stays the same.
Christmas tree reticles like H59, CCH reticle can be used for holdover, you hold the center of the reticle XX moa/mil above the target (you can also hold for wind by hold left/right), this way you can engage the target faster, but require more practice.
