CBS stands for Caster BaSe. ERK is the rack series that these fit. The caster base adds wheels to the bottom of an ERK rack. You must get the base made for the depth of cabinet you have. (20 or 25 inches). There are two types of swivel wheels available, commercial grade (very tough) and "fine floor" designed with a soft rubber wheel which protects your flooring.
Caster bases have a metal skirt that hides the casters.
Note that a caster base raises the rack by 3 3/4".