Sterile vs ultra-pure

It has been my impression that the water purity of NxStage’s Pureflow is considered ultra-pure, whereas their bagged therapy solution is considered sterile. Can anyone explain which method is safest and why?

I would think sterile would be considered safer, but I’ve been using the ultra-pure Pureflow for over six months and I’ve never had a problem with infections or sickness from it.