A plex is a crosshair, + . A BDC is a ballistic drop compensator, with dots/lines/ or circles below the center of the crosshair. This allows one to use the lines/dots/circles to accurately shoot longer distances. A standard crosshair is fine for shooting out to 400 yds, but a BDC or turret is better for longer distances. I prefer the turret style, which lowers or raises the crosshairs for shooting. Usually in MOA.