In addition to rating movies, the MPAA also determines what trailers are suitable for general audiences. If they find that a trailer has offensive content, it get a "red band," which makes it suitable for exhibition only before films with an R or NC-17 rating. But the problem with that system is that "red band" trailers become stigmatized in the same way as NC-17-rated or unrated movies: Many major theater chains will refuse to show them.
After its trailer for Deliver Us From Evil—a documentary about a pedophile priest who was protected by the church for over 30 years—got red-banded by the MPAA for its explicit mentions of child molestation, Lionsgate has now decided to release the film into theaters unrated. Since Lionsgate is an independent company (and thus not bound by the requirement of MPAA member companies), it can release the film however it pleases, but don't expect to see it at your local AMC or Loews theater any time soon.