I know that each one represents a 4x4 transformation matrix (thus the "Mat" part of the name). They have to do with the blending zones for the joint parameters.

There's a really thorough breakdown of the CR2 format at Kuroyume's Development Zone, and there's a great book by BL Render (which I highly recommend) that gives a lot of really useful information on the CR2 file and other goodies inside Poser.

Captain Jack