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