LR Duplex is a simple aimpoint system to assist in longer range shooting, a regular central aim point at crosshair zero'ed at 200 and 3 hold-over points calibrated at 100 yard incriments out to 500 yrds. I guess the easiest way to explain it is that the small dots above the post and below the duplex become your point of impact at distance, an objective at 300 yards would require the first dot below the duplex as the hold on objective point.