There are texture files. A mat pose would not work if it didn’t have the texture files they reference.
DS4.5 and up now use duf files rather than the older file formats. The Genesis 2 Female/Male and Generation 6 line require their textures to be in duf format. That is the reason the V4 to G2Female and M4 to G2Male products were created. These will help you transfer you older Gen 4 products to the newer line.
If you are looking for the texture files, go to the My Library folder where you installed your DS content. There should be a folder marked content. Inside that folder is a Runtime folder. Inside the runtime folder is folder labelled content. By default, may texture files install to this folder as that is the default folder mat poses reference.
You can do UV swapping in DS. G2Male and M6 use the M5 mapping by default. You need the this to use your M4 textures on M6
This will auto convert
I know you can go from M4 to Genesis to M6 but I don’t honestly know if you can go from M6 down to M4. Never tried it.
Odd question but are you using DS or Poser? That can make a difference as well.