With a FFP scope, the reticle and target increase/decrease with changes in magnification. If you had a mil-dot scope, the subtensions don't change with the magnification. On a SFP scope, the reticle size stays the same while the target inceases/decreases with changes in magnification. If you had a mil-dot scope, the subtensions change with the magnification. For example - with a FFP scope a mil is always 3.6"@100 yards. With a SFP scope a mil is 3.6"@100 yards on just one magnification - usually the highest power. Hope this helps.
Tim