What is this number/identifier on a PS3 disc?
It's the Manufacturing Code:
Printed on the discs or packaging of Sony DADC-manufactured CDs are codes indicating master copies (matrix numbers) of discs. These codes begin with a 4-letter prefix followed by a series of digits.
In your case:
BPDL - PlayStation 3 Blu-ray Disc Dual-layer titles
As I cannot find any other information, I'm guessing the six-digit suffix is referring to the specific title.