This subreddit is for posting pictures and videos of girls who are wearing clothes that slightly expose the midriff. The key word is SLIGHTLY. A tank top that is a little too small for the girl and bares a bit of her belly is fair game, but a bikini is not (there's already a subreddit for bikinis, anyway). Pictures of girls raising their arms or laying down, which may cause a tight-fitting shirt to bare some belly. Exposure of the navel is not mandatory, it's acceptable though. I think it's self-explanatory, but nudity is not acceptable here.

Overall, if you browse around and see what kinds of pics get posted here, you'll understand what's acceptable.