Many Face Groove tools have curved holders. Due to the curvature of the holders the tools have a limited set of diameters at which they can plunge. The image below shows the curved shape of the supporting holder.
These two diameters are the minimum and maximum diameters between which the tool can plunge.
The tool inside edge of the groove must be between the Min plunge diameter and Max plunge diameter if the Groove is being cut in the positive direction. The outside edge must be between these two diameters if the Groove is being cut in the negative direction.