DVD9 (or D9) means Dual-layer single sided disc

DVD5(or D5) means Single-layer single sided disc

There are also DVD10 and DVD18.

The capacity of DVD9 is nearly double of DVD5 and can hold more data.

The term DVD9 and DVD5 are used worldwide.

In Europe, many DVD distributors put DVD5 or DVD9 in specification that printed on product (usually on back).

See examples below, French Studio Canal's Angel Heart and MGM's region 3 release of Silence of the Lamb

In China, pirated DVDs always emphasize their superior quality by put DVD-9 on front cover for years. As a result, Chinese consumers get used to this term and believe that DVD-9 means superior. Original studio release such as CAV Warner now also put DVD-9 on cover for their double-layer single sided disc product marketed for China in order to please Chinese consumers. The terms DVD-9 or DVD-5 don't mean pirated copies at all.

The only way to check a disc is authentic or not is by checking the disc inner ring. The SID code (source id) in format IFPI something,  tells you that this disc is mastered by an authorized disc manufacturer. Pirated disc never carry a sid code.