Some theaters wouldn’t exhibit NC-17 films, and some stores wouldn’t sell them on home video.
Before it was essentially abandoned, though, the NC-17 did get applied to some very good films.
By the end of the 1980s, the X rating — originally created by the Motion Picture Association of America to simply denote any film for a purely adult audience — was broken.
In an attempt to create a new adult rating that wasn’t automatically linked with porn, the MPAA introduced the NC-17 rating in 1990 as the replacement for X. While the MPAA succeeded in creating an adult rating apart from pornography, they never quite divorced the idea of adult movies from the stigma of sensational sex and violence.