As merchants accept pre-orders on the future inventory, the pre-order products have zero inventory in HotWax Commerce as well as Shopify. 

If order splitting is enabled when a customer places a mixed cart order with one Pre-Order product and an in-stock product, the Pre-Order item goes to the Pre-Order queue until the item is available, and the in-stock item gets brokered and fulfilled. 

However, when order splitting is disabled, both items go into the Pre-Order queue. Giving the retailer a choice to decide if they want to reserve inventory for the in-stock item or not.

Keep in mind if they reserve inventory for the in-stock product, there is a chance that the merchant could lose orders that need to be fulfilled immediately. However, if they don't reserve inventory, there is also a risk of promising all the inventory to other orders and products going out of stock. So, when the time comes to fulfill the order in the Pre-Order queue, a Pre-Ordered item can be fulfilled, but another item cannot be fulfilled because the product is now out of stock which was originally in stock when the order was taken. 

When these situations occur, retailers will make decisions on an operational level:

• To sell products that are available throughout the year, for example, basic white t-shirts. Retailers don't mind overselling any of the products in the primary assortment. So, if the in-stock product is in the Pre-Order queue, it’s always an available product. Retailers don't recommend reserving the inventory and allocating the available inventory to regular orders that need to be fulfilled immediately. 
• Some retailers sell exclusive products that are not always available or sold throughout the year, which means there is a limited inventory for those items. So if the in-stock product is in the Pre-Order queue, it is an exclusive product with limited inventory. In this case, retailers prefer to reserve inventory for their product assortment.

In this update, HotWax Commerce added a new “Reserve Inventory” configuration that gives retailers the ability to choose whether they want to reduce inventory for an in-stock item that is in the Pre-Order queue or not.