[Ifeffit] difference between Zn-Zn/Fe and Zn-Fe paths

Dear All, Is there a difference between fitting a second shell with two separate paths (Zn-Zn path and a Zn-Fe path) each with its own parameters and fitting that second shell with a combination of two paths (Zn-Zn/Fe)? When would I choose to do that latter, and how? I hope I made myself clear! Thanks, Hashem Stietiya Louisiana State University