The rating system on The Duck is there so readers can get a general idea of the content of a given comic.
E, T+, and M rated comics are accessible to everyone whether they are logged in or not. A rated comics are only accessible to logged-in users aged 18 and over.
You can set the rating of your comic in your Edit Comic area. It is in a comic owner's best interest to set an accurate rating, so their readers know what to expect. If a moderator or admin feels the rating for a comic is inappropriate, they may change it – generally they will ask the comic owner to change it themselves, unless it is an extreme case.