A scope with a second focal plane (SFP) reticle size never changes and stays the same size through all magnification ranges. The Atibal XP8 1-8x24 (SFP) is a perfect example of a second focal plane scope.
A first focal plane scope the reticle is placed in front of the magnification lens. As a result, the scope’s reticle size changes as you shift through magnification. A great example of a first focal plane scope is the Atibal X -10x30 FFP.
Is where the focal plane of the object in the scope is offset from the reticle. If you have parallax, you basically have an optical illusion of sorts that must be corrected. It’s important to understand that parallax should not be confused with the focus or power of the scope. Many LPVO's on the market lack the ability to adjust the parallax with the either a side focus or objective (AO) focus to set the parallax for the distance which you are shooting at. Optics that lack a parallax focus can still be used accurately as long as you maintain proper cheek weld and eye relief. The Atibal HYBRID 12 1-12x32 (SFP) is a perfect example of an optics that utilizes a side parallax in LPVO optic.