A pre-order is used for the products that haven’t been released yet. A backorder option is available for items that are out-of-stock, but have been in stock lately.
Magento doesn’t have a pre-order feature by default. So to make it work on your website, you need either an extension for that or a customization.
To enable backorders, jump to Stores – Configuration – Catalog – Inventory (or per product).
Backorders functionality in Magento allows you to set one of the three available options.
