Riflescopes with a reticle placed in their second focal plane (SFP) are the predominant type of rifle scopes on the market. Second focal plane scopes have the reticle placed in the eyepiece, at the so-called second optical focal plane.
Such arrangement produces the effect that the reticle is seen the same size at all magnification settings. You can clearly see this effect in the picture below:
First focal plane and second focal plane reticle position
Second focal plane rifle scopes are especially popular since their reticles are thin on higher magnification and offer greater precision. Second focal plane reticles also have the advantage that, at low magnification, they are thicker compared to First focal plane reticles and easier to see. These two main benefits are the reason Second focal plane rifle scopes gained such popularity.
There are, however, also some drawbacks of rifle scopes with such optical construction. Reticle subtensions change with the change of magnification, and because of that, distance calculations with the reticle are only possible at one specific magnification setting. Similarly, it is possible to use holdovers for correction of bullet drop only at one specific magnification, which makes this technique very difficult to use. The last, but minor, a drawback is that low-quality riflescope can have a shift of the point of impact with the magnification change.