I might be too stupid, but so far, I can't see any cells being involved when it comes to this method...

> However, we have to tailor the interfaces to what we actually need, we cannot add all possible variations.

True, but we also don't need to restrict unnecessarily ourselves with unfortunate abstractions...

> Since you have already put the ability for the PvtInterface classes to contain multiple tables, I think that is the correct place for it.

placing it there creates a implicit but quite strong API-level dependence on the chosen discretization (i.e., DOFs must be per cell) and what you can pass to the property calculation functions (namely quantities for cells). Since the tables don't induce this dependence per-se this would make these classes quite a bit versatile. Also you would have to add an array which contains the cell indices you want to calculate to the relevant methods anyway, i.e. your cell-based approach would not even be simpler than passing PVT table indices directly...