S5 is local breakout. S8 is home-routed. Basically I think it means that a UE's subscription is such that it can connect to either, all of its "home" PDNs , or "visited" PDNs. The HSS manages the subscription. Now if the UE has to be supported a mix of "home" and "visited" PDNs, it has to be managed by both "home" and "visited" HSS, which is not a practical scenario. My thoughts, of course!