It is the coordinate of the spot reference point on the image plane.  In your diagram you can see that the reference point is listed as the chief ray. The reference point can be set to other values including the centroid, middle, or vertex in the Refer To field of the settings. There is a discussion of this in the help files which can be accessed by clicking the ? at the top right of the settings page. Since the system is radially symmetric only one coordinate is needed.

Thank you so much @David. Really appreciate your quick explanation.