There are different types of MAT file. For mt5 (single) 'MATerial', or mc6 'MATerial Collection', in the Poser Material Room, save either a single material, or a material collection, respectively. To make a 'MAT Pose' (pz2) file in P6 or above, save an mc6 (MATerial Collection), then open it in a text editor, replace the string "mtlCollection"  near the top of the file, with the string "figure". Save the file to a Pose folder with a ".pz2" file extension.