If I remember correctly from my old geometry and drafting classes, the object Andrea shows in his illustration is an "isometric ellipse" rather than the true ellipse drawn by AutoCAD. The isometric ellipse is what was used as a shortcut when doing isometric views with a compass. I do not believe there is any portion of a true ellipse that is an arc segment.
That said, as for your original question, I have no idea what is meant by major and minor radii of an ellipse. I have only see them drawn by methods using Kerry's major and minor axis references.