RCE stands for Region Coding Enhancement.

Region Coding Enhancement is an extra layer of protection along with Region Coding added by some studios to their Region 1 (R1) DVDs to prevent them from being played back in region free or multi-region DVD players. If you attempt to play an R1 DVD that contains RCE protection in a multi-region DVD player, you will be greeted by a map showing where all 8 different Region Codes apply and a message telling you that the disc is intended for play on non-modified Region 1 DVD players.