How does the layout of a flex differ from a rigid?

 How does the layout of a flex differ from a rigid?  I see on a PCB manufacturers website I need to do a stiffener?  Also indicate base material, copper thickness, cover layer thickness, as well as stiffener material and thickness.  Any help anyone can give or point to that will help me figure out how to do flex layouts would be very apprciated.

                                                       rigid flexible PCB