There are a lot of threads about this on the forums, so doing a forum-search for NSFW will turn up a lot of insight from how people on here make the distinction!
From the Tapastic Content Policy:
Adult Material: Tapastic ALLOWS mature content featuring nudity, non-graphic sex, extreme violence, and strong language as long as it is behind our NSFW filter.
If it is one of those things, it should go behind a NSFW filter -- and that's all we know for sure. There's still a lot of uncertainty on exactly where those lines are, and different people use the filter differently.... but NotSafeForWork doesn't mean "not safe for kids." It means there's adult material you might get in trouble for looking at.
EDIT: here's one recent thread on when violence should be NSFW!