The repeatability is a measure of how precise a target position can be reached multiple times from one direction (uni-directional) or from multiple directions (bi-directional). The bi-directional repeatability is mostly dominated by the reversal error of the system.
The uni-directional repeatability is calculated from the standard deviation of the positioning error at each target position along the full travel of the motion system.
The reversal error is given as the difference in the measured mean positioning error when changing the approach direction toward a specific target position.
The repeatability of a motion system may depend on the point of interest of the application i.e., the distance from the end effector.