Difference between volume.cs and PostProcessingVolume.cs

Unity 2020.3, URP
Hi, I have 2 post-processing in my project.
One uses the Volume.cs script located in Core RP package and does not require anything else(like layers), and it works.
The second one is Post-ProcessVolume.cs in the Post-Processing package, along with Post-ProcessLayer.cs and so on, and doesn’t work at all
Help me figure it out please