As of now (2016-02-10 20:04 UTC -6), there are 4 questions tagged with world-building, but it's not clear to me what topic or category it defines, based on those (4) questions it has been used.
I imagine it used when somebody asks about "hypothetical or fictional planets, or alternate Earths". Still, it might not be the most appropriate tag, since I suppose (rather, speculate) that the motivation for most of such questions is not necessarily "world building" in the sense of, e.g., a science-fiction work, but more for modeling or simple curiosity.
And what about some homeworks? (e.g., this one) Should they be tagged world-building as well?