This mod allows you to backorder items that have been ordered but are out of stock.

When an order has been placed but one of the items is out of stock, you can view the order details and click "Backorder this Product" for the product that is out of stock. The product will be marked as backordered and will be removed from the original order and a new order will be created containing just that product. You then have two separate orders, so the original order can be dispatched without the out of stock product, and the new order can be dispatched later when the product is restocked.

The new order that is created will have a status of "Backorder", thus allowing you to keep track of all orders which are waiting on stock to be replenished.