This article explains the different tools provided by Connect to handle backorders (sales order for which at least one product is not in stock).
When a new order comes without stock, you can easily identify missing products from the sales > order menu, they are in red :
In the order preparation, orders missing stock are in the "Backorder tab" :
Product to purchase for backorders are displayed in the supply needs screen (menu Purchasing > replenishment) :
From the supply needs screen, you can easily create a new purchase order with backorder products and notify the supplier.
Once the product are delivered to your warehouse, you must create a purchase order reception which will put the product in stock.
At this time, Connect will automatically assign the incoming stock to backorders :
Also, this order will move to the "In stock" tab in order preparation, meaning that you can ship it :