Your order contains two numbers: an SBP number and a BP number.
BP number: relates to the individual product.
SBP number: relates to the entire order.
For example, if you have ordered a landscape photo book and a square photo book in a single order, the number relating to the entire order would be the SBP number. The landscape book would have a BP number and the square book would have a (different) BP number.
If you have ordered multiple items of the same product, then you will only have one SBP and one BP number.