How is quaternion that returned by gym.acquire_rigid_body_state_tensor defined?

[Xinqi-L]

I need positions and rotations of rigid body states of Humanoid. But I wonder if positions and rotations are defined relative to the root of humanoid(like Pelvis?) or the world?
It puzzled me a lot when trying to convert the behavior of humanoid to a <.bvh> file. Any suggestions for converting into .bvh?