Backorders come from three places: a product sells faster than planned, a supplier is late, or a store chooses to sell ahead of a delivery it knows is coming. In Shopify they usually come from one setting in a product's inventory section, Continue selling when out of stock, which lets the stock level go below zero instead of closing the product to new orders.
It is a real alternative to losing the demand, and it works exactly as well as the communication around it. An honest date at checkout, kept, is tolerated by most customers; a vague one, missed, produces cancellations, refunds and disputes that cost far more than the sale was worth.
On a nouz statement a backordered order counts on the day it was placed, like every order: its revenue, its cost of goods and its parcel all land on the order date, so the statement never splits it across months. What it changes is stock. The units sold below zero are already owed to customers, so the reorder planner on Insights, Inventory counts them into the next order.
It also shifts the cash calendar in your favour: the customer funds the purchase before the supplier is paid. That is genuinely useful and quietly dangerous, because it works until the shipment is late and then several promises come due at once.