NC-17 a new rating back in 1990

They should release two versions of the movies, the original adult ones and also ones aimed at a younger audience.

Did it replace the X-rating or was it a way for the MPAA to distinguish between "artistic" X and porno X?

When movies (or moving pictures as they were called back in the day) were a brand new concept, there was no such thing as a rating system.

This era in the history of movies was known as the "Pre-Code" era.

Then in 1936, the Hays Code was introduced, which was a series of rules regarding what could and could not be shown in a movie.

This code became a mainstay until 1968 when it was replaced by the MPAA rating system, which had four types of ratings -- G, (for general audiences) M, (for mature audiences) R, (restricted to those 19 and up) and X (which is for adults only)

Eventually, M got changed to PG, (parental guidance) and PG-13 was added in 1984 as a way of splitting the difference between a PG and an R rating.

And the rest... Is history.

it didn't replace anything. it was a new designation to put between the existing 'pg13' and 'r'. so where r might cover some level of sex related things, pg13 was inteneded to cover things too strong or violent for young kids, but would be fine for most teenagers, nc17 was supposed to be another step up in violence, gore, mature themes, etc before r. x has always been for explicit sex.