It's just the machines being used by Davita that they bought. And Davita became in effect a franchisee of NxStage if FMC wanted to use the System One they'd have to get them through Davita. Existing contracts and non-profits are not affected by this deal.
Epoman, I share your skepticism of Davita's motives in all things even as I acknowledge it is almost always a mistake to attribute motivations. If 7% leads to 51% then that would raise questions. 7% is more a vote of confidence. Davits's ownership percentage will be an interesting number to watch.