Your order contains two numbers; an AL number and a SAL number.
AL number: relates to the individual product.
SAL 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 SAL number. The landscape book would have a AL number and the square book would have a (different) AL number.
If you have ordered multiple items of the same product, then you will only have one SAL and one AL number.