Attached Link: http://www.morphography.uk.vu/fileguide_1.html

Above is a link to a page on English Bob's site, you should find most of what you need to know there. The word "mat" is used in a few diffrent sences, sometimes it is just used as an abriviation for the word material. A "*.mat" file is one associated with an "*.obj" file, it contains information about what materials to use for the object. There is also so a type of "*.pz2" called a MAT Pose, it is loaded from a pallet on the rigth hand side of the Poser interface, and is used to apply textures and/or colours to to figures and props.