If Shopify keeps track of your inventory, quantities of any out-of-stock items that you do not allow customers to order when they’re out of stock will have their quantities adjusted in the cart to prevent the ordering of out-of-stock items. This is essential to the functionality of wholesale order processing and cannot be changed (if you do not have Shopify track your inventory then this feature is automatically disabled)
*The following two features only work if Shopify keeps track of your inventory. If you do not use Shopify to track your inventory, we highly recommend keeping these features off. If Shopify does keep track of your inventory, we highly recommend turning these great features on!
If a customer has out-of-stock items in their cart and you have selected Allow customers to purchase this product when it's out of stock in the Shopify admin, enabling the Ship Partial Option will allow the customer to choose whether they want their order shipped complete, or in parts as available. Their choice will be added to the notes section of their order.
If a customer has out-of-stock items in their cart and you have selected "Allow customers to purchase this product when it's out of stock" in the Shopify admin, enabling the Backordered Message will place backordered notifications on the product throughout your site. Backordered messages will appear on the product's page (right above the description), and on the Quick Order Page as well. There will also be a link in the Ship Partial option on the cart page (if enabled) for customers to view a list of backordered items in their cart. Additionally retail customers will see a backordered notice on the product's page.
If you use Shopify Transfers feature and have set an Expected Arrival Date for the product, all backordered messages will have the product's expected ship date included. If you don't use transfers they are a really powerful feature Shopify offers to help with tracking incoming inventory. Learn more about transfers.