All products have two attributes regarding backorders.
In addition, there are also site settings, accessible to installation and site administrators, that can override these and not allow any backorders, or limits the number of backorders allowed.
How your backorders are filled once you have received additional stock depends on how your warehousing, inventory and shipping are set up.
On the front end, the product detail page and the cart pages will show an expected availability date by pulling data from your restocking information. This will happened automatically if you have GWS. If there is no data, this will be blank. If you have manual warehousing, you can create re-stockings to provide the information, which will then display on the front end.
If any items in an order are backordered, the entire order shipping (individual destination) will be held pending the arrival of the backordered items. To deal with this, Commerce creates multiple order shipments within the order to ensure the in-stock items reach the customer as soon as possible. CSRs could also split the order entirely, if desired.
Backorders are not partially filled automatically. If a customer orders three of an item and only two are in stock, the order will not ship. (A CSR can go in and send the two items in stock, if desired, and if there is some stock left.). The orders will be split into separate shippings and manually capture payment to ship available products. CSRs will also need to notify the customer manually in this case.
Product pages and order pages should list when items are available.
Payment is not captured until a backordered item is shipped. If a payment capture fails for any reason, such as the authorization expiring or an invalid credit card, the system will process an authorization and try again to capture the payment. If it fails again, the order will not ship, and it will be skipped in the auto-fulfillment process. The order will be placed on Manual Hold, a Manual Hold email will be sent to the CSR and a notice placed in the Order Notes. A CSR will have to contact the customer regarding payment or cancel the order.
When the item comes back into stock, if you have GWS, the system will fulfill the order, capture payment, set up shipping and send an email to the customer. For manual inventory, the order status will change to OK, but the order will still need to be fulfilled manually.
If you do not have GWS integration than backorders MUST be fulfilled by a CSR. When the stock comes into order, the order status will change from backordered to OK, but the shipping must be manually set into motion. You can filter by status or check the backorders report to see what orders must be fulfilled.
See "SKU-Specific Product Information" and See "Adding a Product to the Catalog" for more about backordered product attributes.