I started building a collection of 4k discs and I wondered why most of these along with 1080P Blu ray do not have a reference number. I then got a a hold of a few that do. The ones that do have reference number tend to be Sony,Disney,Universal produced movies. Every single HD DVD disc in my collection has a reference number, so what made Blu ray drop the ball ? Just saying.
As I started looking back through discs I did come to see some Blu rays that did have the reference number. just a few. I like inputting the number when I catalog movies as I receive them. I'm so glad that LDDB can use the reference number for Laser discs and HD DVD when searching for those.