Your order contains two numbers: an SNO number and a NO number.
NO number: relates to the individual product.
SNO 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 SNO number. The landscape book would have a NO number and the square book would have a (different) NO number.
If you have ordered multiple items of the same product, then you will only have one SNO and one NO number.