DVD+R is able take advantage of the not-so-well known bitsetting, also called bit setting, bit mode compatibility, compatiblity bitsetting and probably some names I haven't heard of yet.
Whatever you call it, bitsetting is a process that changes the identity of a DVD+R disk as seen by DVD players. DVD-R disks have information embedded in them which identifies them to players as DVD-R. Replicated DVD's (ones that have been stamped rather than burned) are identified as DVD-ROM. DVD+R disks have no such embedded identity and, using certain DVD burners, it is possible to make the DVD+R disks identify themselves as DVD-ROM.
Certain older DVD players will not load DVD-R or DVD+R disks at all because those formats did not even exist when the player's firmware was written. However, by changing the bitsetting on a DVD+R to DVD-ROM, those players should be able to load and play the DVD+R.