My bet is that's 8.5 Billion Bytes. However don't use these for booting. Too many machines cough on that.
I've got some of these double layer DVD-R discs. I had to make copies of a disc I got from Dell with more than 4.7 GB on it. It's nice that these have greater capacity, but avoid them if you can because as Bob suggests, these are very finicky. When I tried making copies at default speeds, it crapped out in the middle of the copies. I had to slow it down to 2X to get it to work.
They still make dual layer discs. See this link
I've got a box of them, in my case made by Memorex. They hold 8.5 GB according to the printing on the face of the box.
It was newegg, actually.
I just assumed that was a standard advertising. Kind of like how USB drives will advertise a certain number of gigabytes, but the actual amount of storage is lower, because 64GB advertised means precisely 64,000,000,000 bytes, whereas the computer considers 64GB to mean 68,719,476,736. I just assumed it was the sort off thing you need to be aware of.
Also, what's BD? I know what BR is (Blu-Ray), but not BD.