FFP Brief Introduction: Most scopes have the reticle fitted in the second focal plane (near the eyepiece). While, presently it has always been the practice to fit the reticle in the first focal plane (the reticle increases its size when change the magnification from low to high). Each system has its advantages
The advantage with telemetric reticles (like rangefinder & Mil-dot etc) is that the target image and distance between dots remains constant even when changing magnification. It is the system that is widely used by the TOP military suppliers now.