I was going to add a note into my translation on this phrase which occurs about a dozen times in the Daoxing. It may well be worth comparing passages which contain this phrase in the Daoxing with the extant Sanskrit. My hunch is that it will probably be a short-hand version of 'gods, men and asura' and similar lists of sentients. The term 天下, is well established meaning 'everywhere', in the sense of 'the world'. 天上 does occur in a number of pre-qin texts as denoting something from above, but the compound 天上天下 is not found.