Understanding surface and volume polarization charges
Dielectric polarization happens from polar molecules. The work done by an external field behaves like a charge, but unlike a electrons in a material, these charges cannot move. So if you had some dielectric material, you'd want to be able to find out what an external electric field does to it.

\$P_{pv}\$ is the bound charge, its locked up inside of the polar molecules, they aren't going to escape the material and move down a wire.

\$P_{ps}\$ is the surface charge. There could be charges on the surface, these need to be accounted for.

You add them both up and you know how much charge you have ( and what kind of useful things you can do with it like store energy)