the assumption here is that all pending approval sales orders will show up as back orders, even if you have sufficient stock to currently fulfill them.
If you needed to know whether a particular pending approval-sales order line
would be back ordered, you'd need to use a more complicated formula like a
Cumulative/Running Sum, ordered the same as your Accounting Preference "Item Commitment Transaction Ordering" (i.e. by "transaction date", "order priority", or "expected ship date")