DVD layer 9 和普通DVD有甚麼分別?

I think you mean Dual Layer DVD and normal DVD... don't mix up D9 & D5.
DVD volume size is 4.7 GB, but dual layer volume size is 8.5GB... that mean you can store more data inside... if the DVD is mention D9 format, that mean the disk is use Dual Layer Disk to burn, being D9 mean high quality and big data size of the movie, that why, if the movie as D9 format, that mean the quality SHOULD be better than use D5 disk... at least the compress rate is lower and get better quality!

簡單講DVD單面單層的容量是4.7G,而DVD9即是單面双層容量是8.5G,等於單面單層的兩倍,所以用DVD9錄製的效果會較好。