It essentially related to the ability to access money in a 401k from overseas if we were ever to make the move back across the pond. There could be double taxation issues once we made the move unless we fail to declare the 401k in the UK, which isn't really an option. It's all very complicated, and in all honesty I'm still trying to get my head round it, but it certainly seems to be more trouble than it's worth unless we plan to retire in the US, which we probably won't.