Reticle placement within the erector tube determines if the riflescope is a First or Second Focal Plane. The erector assembly is a critical assembly within the riflescope and is what is responsible for magnifying or de-magnifying the image.
A reticle in the First Focal Plane is placed at the front of the erector assembly and remains in the same visual proportion to the target across the riflescopes entire magnification range. While the proportions of the reticle will appear to change when adjusting the magnification, in reality the reticles values are remaining in proportion to the target. This is beneficial for rapid ranging and holdovers at various distances and magnifications.
A reticle in the Second Focal Plane is placed at the rear of the erector assembly will appear to stay constant in size across the magnification range. The reticle subtensions are set at a pre-determined magnification change as magnification changes. Simple math can be used to determine the change in values, yet the reticle line size will remain consistent for a fine aiming point regardless of magnification.