A .mc6 is basically a collection of textures that cover multiple material zones in a figure/prop and only be applied to that figure/prop or similar. A .mt5 usually is applicable to just material zone, so if you attempt to apply that file to a multi-material zone object, Poser will ask you which face to apply it to. You can still create a .mc6 file out of a procedural shader for a particular object. You should get a dialog box for selection when you initiate the save option.