Parallax adj. (adjustment) range tells you the minimum and maximum distance you can adjust the parallax on your rifle scope.
Most rifle scopes (mostly) with maximum magnification 12 x have parallax setting fixed, for example, to 100m (EU 100 m / US 100 yards or 91 m). On those, you can’t make any adjustment with parallax.
Rifle scopes with more than 12 magnification zoom have adjustable parallax setting, for example, from 50m to infinity.
There are some rifle scopes with special parallax settings. Schmidt & Bender Exos rifle scopes have CC mode, which means when you turn the magnification on 1, the parallax changes simultaneously from 100 meters to 25 meters, so it is much easier to look with both eyes open and therefore easier to shoot close distance running targets.
Source: Revija Lovec