VTX
From Valve Developer Community
VTX is the extension for Source's proprietary mesh strip format. It stores hardware optimized material, skinning and triangle strip/fan information for each LOD of each mesh in the MDL. Currently it is found in .sw.vtx (Software), .dx80.vtx (DirectX 8.0) .dx90.vtx (DirectX 9.0) and .xbox.vtx (XBox) flavours.
