notation

There are so many different understandings of multisite, and so many different solutions with configuration options, that I wonder if we might make a way of quickly describing them. As I see it multi-site solutions differ mainly on what is shared between sites, and what is unique. Therefore, here is a rough first cut of shared / unique features against a range of solutions. I don't know enough to fill in the boxes - I'm only suggesting a way of notating here.