First off, to understand the differences between Front Focal Plane and Rear Focal Plane (Also called First Focal Plane [FFP] and Second Focal Plane [RFP or SFP], respectively) scopes, you should understand the basic mechanics of variable scopes.
In a variable scope, there is a main tube, and within this main tube resides another tube called an erector assembly. This tube extends from the power selector ring to just in front of your adjustment saddles. In the erector assembly resides a pair of lenses called erector cells. These cells move closer together or farther apart when the power selector ring is manipulated, effectively changing your magnification.
This is important because it sets up the physical difference between Front Focal and Rear Focal Plane Scopes. In Front Focal Plane Scopes, the reticle is installed on the front of the erector assembly, so as the power selector ring is turned, changing the target magnification, the reticle is magnified proportionally to the target. Simply stated, if you are using a 4.5-14 power scope, and you are looking at a target at 4.5 power, you have magnified the target 4.5 times. On a FFP scope you are also magnifying the reticle 4.5 times. If you change the power to 14, you are now magnifying the target 14 times, and the reticle is also magnified 14 times.
Another way to describe this is that anything IN FRONT of the erector assembly is magnified, while anything rear of the erector assembly is not. This is why I prefer calling the different focal planes either Front Focal Plane, or Rear Focal plane respectively.
The two locations in the scope where the reticles are placed, are also the only locations within the scope where the image resides in focus, BESIDES where it exits the eyepiece. This is why the reticles are placed in these locations, otherwise we wouldn’t even be able to see the reticle in the scope.
While it’s sometimes said that a FFP scope has the reticle grow and shrink during magnification changes, the reality is the reticle stays the same size in relationship to the target throughout the power range, because it is being magnified at the same value as the target.
Conversely, in a Second or Rear Focal Plane scope (SFP/RFP), the reticle is located in the rear of the erector assembly, so as the magnification of the target is changed, the magnification of the reticle appears to not change.