Your order contains two numbers: an SAL number and an AL 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 one order, the number for the entire order is the SAL number. The landscape photo book will have an AL number and the square book will have an (other) AL number.
If you have ordered multiple items of the same product, your order will only have one SAL and one AL number.