This is how it was explained to me-
"The parallax adjustment only helps reduce the error of the reticle
placement on the target when your eye is not in the center of the optics.
If you are highly skilled at placing your eye in the center, and can hold
it there, you can then turn the parallax knob and see if you are getting
reticle shift. Reticle shift occurs when there is a slight misalignment
in the lens group or misalignment between lens groups. None of this
matters much inside 200 yards where most guys actually shoot and practice.
Since I shoot mainly at 600 in practice and beyond when out in the field,
it matter a great deal to me.
If you are not skilled at centering your eye in the scope, that is no
problem when trying to proof a scope's alignment. All you have to do is
set the parallax knob on one setting and move your eye up and down looking
for the highest point the reticle reaches and the lowest point it reaches
on the target. It should be easy for your eye to find the midpoint of
those two extremes. Now, move the parallax knob to a slightly different
setting, and repeat the vertical movement of your eye, once again noting
the extremes. With most scopes you will notice not only a vertical shift
of the crosshair on the target, but a substantial horizontal shift as
well. This is not acceptable for a long range scope.
Our HRT team had 5 Leupolds, we had problems with them holding Zeros and with the parallax knobs causing point of aim shifts. We switched them to the Nightforce nxs models and both problems went away."
Lightforce Lights Dealer