I think you may indeed be talking a little past each other. In any case, changing the high-level-interfaces/classes (BlackoilPropertiesInterface, BlackoilPropertiesAdFromDeck etc.) to take region indices instead of cells is *not* what I had in mind, or thought @andlaus wanted to do. We are talking about which class should perform the cells to regions mapping (BoPAdFD or PvtInterface for example), it should never leak out through the top interfaces (into for example FullyImplicitBlackoilSolver).