The Back-Orders page lists all orders with inadequate inventory to ship the order completely.
If there is not sufficient inventory available for an order your options are:
WD cannot split orders but you are welcome to split orders. The portion of the order that has available inventory can be sent to WD for processing. The portion without inventory at the WD Fulfillment Center can be held by you until inventory is received and then submitted to WD for processing. Note: this option results in increased shipping costs due to multiple packages being shipped.