Racking changes the alignment of the front and back needle beds. Combined with transferring, racking allows the lateral movement of stitches, essential for—among other things—bind-offs, increases, decreases, cabling, lace, and tube rolling.
Racking is generally limited to a few inches of total travel, with the exact limit depending on the machine type and gauge; maximum racking offsets of between 6 and 12 needles are common.
In knitout the racking is set with the rack N operation, where N gives the offset of the back bed relative to the front bed.
So, for example, rack -2 aligns needles f2 and b4.
Fractional rackings which mis-align the front and back beds, allowing, e.g., very dense rib structures to be knit by using all needles on both the front and back bed.
The Shima SWGN2 calls this quarter-pitch racking (corresponding to N values like 1.25 and -0.75).
The Kniterate calls this half-pitch racking (corresponding to N values like 1.5 and -0.5).