Poser uses Pixar's OpenSubDiv. The source including demo code is available on GitHub: https://github.com/PixarAnimationStudios/OpenSubdiv

The detailed description and very detailed descriptions can be found here:https://graphics.pixar.com/opensubdiv/docs/intro.html

(By the way, the documentation is not only interesting for programmers - you can find interesting facts about SubD and what you should pay attention to when using it)

Could it be that the mysterious data has nothing to do with normals or UV, but somehow describes Bezier curves?

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