To play a DVD, the region code of the DVD must match the region code of your DVD drive. Your DVD drive is automatically set to a region code the first time you play a coded DVD. If the region code for a DVD doesn’t match the current region code of your DVD drive, a dialog asks if you want to change the code for the drive.
WARNING: You can set the region code for your DVD drive only five times (including the original setting). The fifth time you change the code, it is permanently set to the last code you set. Any DVDs you play must match the last code you set.
via TJ Luoma