Material definitions determine how different surfaces are rendered visually. Parameters include colored tinting, transparency modes, texture scrolling (for lava), and many more.
In DeS and DS1/R, MTDs are found in .mtdbnds, which are standard BND3s.
There are still many unknown values in MTD, but the overall structure and parameter format is understood for DeS, DS1, DSR, DS2, and DS3, so they can be fully read and written.
See the section title.