It's at the very beginning of the discussion on p.83. There is one Foundation, but each Host 'grows out of' a certain section of it, generates its own portion, its own nodes, that interface between the Foundation and that specific Host, and so they speak of a particular Host's Foundation as though it were a seperate thing, even though it isn't. But going into a Host's Foundation is like the movie 'Inception' - there's that funky dream logic at work, just in that Host's Foundation. It would, I think, be even more different/abstract/intense were you to dive into the deep Foundation itself.