This short article talks about the interpretation of each dimension of the observation space of PyBullet envs obtained using import pybullet_envs.
Let's use HalfCheetahBulletEnv-v0 as an example.
At the time of writing, pybullet_envs can be found here:
https://github.com/bulletphysics/bullet3/tree/master/examples/pybullet/gym/pybullet_envs
If we look into its __init__.py, we see that HalfCheetahBulletEnv-v0 is registered as: