The main difference is that they have different capacities. D9's capacity is larger than D5's. DVD ± R DL discs referred to D9, the single-sided double-layer discs, the maximum capacity is 8.5GB. Ordinary single-sided single-layer DVD discs for the maximum capacity of 4.7GB, referred to as D5.
1) A D5 disc is single side single layer. D5 holds around 4,700,000,000 bytes and that is 4.37 computer GB where 1 kbytes is 1024 bytes.
2) A D9 disc is single side dual layer. D9 holds around 8,540,000,000 bytes and that is 7.95 computer GB.
For commercial DVD, you could insert the DVD disc into your Mac, then press Control key and left-click your mouse at the same time, select Get Info option to check its capacity. If the capacity is less than 4.7 GB, it is a D5 disc. If the capacity is more than 4.7GB while less than 8.5GB, it is a D9 disc. (For details, visit The Differences: DVD5 vs DVD9 vs DVD10 vs DVD18 [Inforgraphic])