Question about material id 6: fluid in `mpm_utils.py` but stationary in `mpm_solver_warp.py`

[worfsmile]

Hi, thanks for wonderful work!
While reading the MPM implementation I noticed something that seems a bit confusing regarding the material ID definitions.
In mpm_utils.py, material ID 6 appears to correspond to fluid:
third_party/PhysGaussian/mpm_solver_warp/mpm_utils.py#L519
However, in mpm_solver_warp.py, material ID 6 is defined as stationary:
third_party/PhysGaussian/mpm_solver_warp/mpm_solver_warp.py#L17
This seems confusing because the same material ID is used for two different meanings.
Are these two definitions related, or are they used in different contexts?
Thanks!