im sure there is a more technical explaination but it means the scope is set up with its point of hit and its cross hair or red dot to be optimum for that range. it doesnt mean that it cant shoot farther or shorter ranges. just optimized for that point of aim. 100 yards seems a bit much for a red dot but im no expert. if your shots are in that range than your good. for muzzleloader or shotgun 75 is my favorite.