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