[tpmx]

Where is the format specification?

https://graphics.pixar.com/usd/docs/

The closest thing I can find there is the glossary and the C++ doxygen api docs.

[jpm48]

It's a schema specification with various plugins. I guess it will mainly be about the geo specification for models (and perhaps ignore the shaders etc). Documentation is hard to come by, you basically have to look at the source / examples.