This refers to the double layer, double side DVD discs with a total capacity of 17 GB, which can store a video data that can be played for 482 minutes.
The double side, double layer disc is composed by two film bases with recording layers sticking together. The production of this Disc has a high requirement on technique; this means a higher producing cost. So the common manufacturer shall not use DVD-18 format if no further requirements. Actually, DVD-18 discs are rare in market.