To understand FFP vs SFP Reticles:

First Focal Plane
These reticles are found in the front of the scope. They change in size as you increase or decrease zoom. The advantage of this reticle, is the subtensions are always the same value no matter the magnification currently in use. March FFP Reticles

Second Focal Plane
These reticles are found in the back of the scope, and remain the same size regardless of the increase or decrease in magnification by the user. The advantage of this reticle style is that the reticle can have ultra-fine markings and still be seen at any magnification. March SFP Reticles