> Isn't this class holding the mapping from cells to regions?

nope, the public interface is not concerned with PVT regions at all if I understood it correctly. You just call `pvtProps.foo(fooContainer, cells, params)` and it calculates the foo property for all cells you requested. the problem with the surface density is that you don't specify the grid cell which you're interested in, so the PVT-region selection can't be done in a transparent way. (or maybe I missed something which is of course always possible.)