What's the mean of timestep.observation.players?

[hilanzy]

>>> dummy_env = envpool.make_dm(task_id="Pong-v5", num_envs=32)
>>> tmp = dummy_env.reset()
>>> tmp.observation.env_id
array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16,
       17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31],
      dtype=int32)
>>> tmp.observation.players
players(env_id=array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16,
       17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31],
      dtype=int32))

What's the mean of timestep.observation.players? Why env_id in players?