When the 'available for preorder' function is enabled on the product in the ERP and 'availability depends on stock' is enabled in Sana, customers are allowed to purchase items on preorder. However, when a customer orders an item that is in stock, but low in stock, the preorder function allows the customer to purchase both items that are in stock and on preorder. The problem with that is that customers have no knowledge of the amount of the product that is in stock vs preorder.
For example, a customer orders 125 units of a product that has 25 in stock. This would mean that they have ordered 25 units of in stock product and 100 items that are on preorder.
It would be best for the customer to receive this information in the checkout so that they are aware that some of the items might delay the order. One option would be to display the information at checkout like '25 items available / 100 items on preorder.' It would be even better if the customer was given an additional option to separate the order with preordered items. This would mean that customer could proceed with the order with in stock items and the preordered items would be put on a separate order.