It is interesting to note that the DL in DVD+R DL actually stands for Double
Layer and not Dual-Layer, however the terms are often interchanged, and some
media is advertised using both terms. The DVD+RW Alliance uses the Double
Layer terminology to help prevent confusion with the fact that most DVD
burners are also capable of reading and writing Dual Format (+ and -) media.
Since the term Dual-Layer was originally used with pressed DVD-9 discs, it
seems that many are maintaining the use of that terminology for pressed
DVD-9 discs, while reserving the Double Layer designation for writable or
rewritable discs. In the end, the only important thing to note is that for
all intents and purposes the terms are identical, and are often used
interchangeably.