It is assigned to films the MPA believes most parents would consider "patently adult" and not suitable for their children under 18.
[1] The rating does not designate films as pornographic or obscene, but simply that the content is suitable only for adults.
[1] The rating may be issued because of violence, sex, drugs, or other elements.
NC-17 originally stood for "No Children Under 17 Admitted" to combat the misconception that the rating indicated a film was pornographic.
In 1996, the MPA reworded the NC-17 rating to "No One 17 and Under Admitted", effectively raising the minimum age for admission from 17 to 18.