It is important that you check this for any 18+ releases or pornographic works (hentai). This is used to filter adult anime if a user has disabled 18+ mode on their profile.
Works that feature sexual content as plot devices or for fan service without it being the central focus of the series should not be marked as adult. For example, series like Yosuga no Sora and Shinmai Maou no Testament are not marked as adult.
However, other series with strong sexual content such as BL (Boys’ Love) and josei focused manga and anime like Rent Boy or Skirt no Naka wa Kedamono Deshita should be marked as adult. Although they don’t fit the typical western definition of hentai, the intent behind them is enough to consider them pornographic.