What is the meaning of the index number in Print Settings

What is the function of the number index in Advanced Print Setting related to the filling options?




Index Button Advanced Print Settings.PNG

Hovering over the option will cause a pop-up which offers explanation. The 'shell' feature will print the part with no horizontal faces, and that number adjusts the thickness of the outside contour. 


Thanks, that explains it, but not the fact than you can choose this option with other infills, bad UI design. Thanks!