Nothing official; not yet, anyway. We've all heard the old Potter Stewart, "I know it when I see it." Nudes aren't necessarily smut, and smut doesn't necessarily have to feature nudes. Here's the loose standard mk shared with me once that I would like to somehow formalize, but don't know how, exactly, to do so: Can anyone reasonably think that this content is "thoughtful"? If not, then we don't feel bad killing it (although we probably won't unless it is porn; that's what "ignore" is for). I will say that NSFW content can be acceptable, but certainly not if it isn't labeled as such.
I don't really like subjective determinations like the one established by Justice Stewart. I believe that mk also used the phrase "thought-provoking" rather than "thoughtful." I'd argue that salacious content can easily apply to a strict definition of either "thought-provoking" or "thoughtful," but I'm not the one making that determination. I'm happy to see hubski emerge as a home for diverse, thoughtful discussion on the Internet. I think the true test of this platform is whether that discussion can coexist with a good dose of smut.
As long as the site is small enough that it can be easily monitored by a few of us, porn will be removed as soon as its noticed. That really isn't negotiable. If it grows to a place where its too big to monitor individual posts, then I suppose it will be up to the community to decide what is seen and what's ignored.