Vortex Optics VIPER XBR 2.5-10X44 CROSSBOW SCOPE - Manual (Page 6)

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Customizable Rotational Stop (CRS)
The elevation turret on the Viper XBR crossbow scope features the
unique CRS rotation stop. After sighting in the crossbow, the CRS
allows you to quickly and easily return to an original zero point
after dialing in arrow drop corrections.
The CRS feature is particularly useful when dialing large multi-
revolution elevation corrections. The CRS allows you to quickly
spin the elevation dial back to the original zero without having to
carefully count revolutions or clicks.
See how to install CRS shims in the Setting the CRS Stop and
Indexing Elevation Dial section on pages 17–18.
CRS Shims
Focus and Parallax
Parallax is a phenomenon that results when the target image does not
quite fall on the same optical plane as the reticle within the scope.
When your eye is not precisely centered in the eyepiece, there can
be apparent movement of the target in relation to the reticle which
can cause a small shift in the point-of-aim. Parallax error is most
problematic for precision shooters using high magnification.
The Viper XBR crossbow scope is factory-focused at 75 yards and will
have minimal parallax at distances from 35 to 125 yards. At distances
closer than 30 yards, there may be a very slight shift of the reticle on
the target (parallax) if your eye is not centered directly behind the
scope’s eyepiece. This shift can be eliminated by keeping your eye
centered behind the scope when shooting.