I still don't really get it (and I've checked both FAQs). As I'm both a professional and hobbyist this answer seems to suggest that I pick the site based on whether I'm working on a client project or a personal project, even though the nature and content of the question could be the same in both cases. Or, to put it another way, describing the difference between the sites based on who they are for (as opposed to what they are for) is not very helpful if you fit both descriptions.