Alright, assuming that there's some sort of dome, or a really high ice wall keeping the atmosphere in, then I would speculate that "FE space" is the same as "RE space". If, however, this is not the case, I'd guess that the "atmosphere" actually permeates the eniture universe (rendering the term "atmosphere" totally inaccurate...of course if we're already assuming a flat eart, it's already a misnomer).