Yet another assimp question

Hello, I'd like to know more about the nature of a certain piece of data in skeletal model formats that we tend to run into making games.

For a while already, I had assimp as a dependency to my framework. I used it with success to import models with geometry and textures, and the mesh hierarchy. I also made skeletal animation work, for the most part (example, all mixamo meshes work perfectly). There's a bit of theory here that I don't understand and I'm suspecting that the reason that SOME meshes aren't working fine for …