This box is only available for these feature types: Square Slot Notch, Polygon.
Square Slots, Notches, and Polygons are not always exactly square. They often have radii instead of corners. The Corner Rad box holds a value the size of that radius. The radius value controls where the hits take place on these features.
For a Square Slot, it is used to determine the location of each hit during the measurement of the slot to avoid hitting on the radii.
For a Notch Slot, hits along the edge opposite the open side will be adjusted off the radius.
For a Polygon, hits along the polygon edge are adjusted from the corners of the feature adjusted off the radius.