Adult DVD's can be played in all 6 regions.
The reason for the regional coding is so that if a movie is being released on DVD in the US it cannot be played in other countries since that would hurt that country's theatrical release (Hollywood movies play the US theatrical circuit long before the rest of the world.). Normally the US video release of a movie is during the world-wide theatrical release.
The regional code was originally developed so that the movie studios could 'black out' overseas markets during the foreign theatrical releases. The DVD players are supposed to decode only DVD discs made for that specific region. The regional code is contained on a 'flagging chip' -- which is easily removed by an electronics or video technician. Once the 'flagging chip' is removed, the DVD player will play any DVD disc from any region.