This property gives the path and name of the .raw file that is used to specify the overall materials of the terrain, with numbers 0-15 (the top 4 bits of each byte are not used):
0 Default 1 Water 2 Dry Grass 3 Juicy Grass 4 Dry Dirt 5 Wet Dirt 6 Mud 7 "You are leaving the battlefield" 8 Gravel 9 Frozen Ground 10 Dry Sand 11 Wet Sand 12 Rock Surface 13 Sand Road 14 Wet Road 15 Paved Road
The heightmap file for the terrain is specified by the File property. The MaterialSize property specifies the dimensions of this image. TexBaseName sets the location of the actual high-resolution color textures.
Source and more details