I cannot find 'Columns section' and certainly have not found 'Backordered' (see below)...
Although you can't track backorders on invoices, you can customize a report to see what items are on backorder by following these steps:
1. Open Reports from the menu and from Sales select Open sales orders by item
2. Enter the Dates and open Customize report
3. From the Columns section in the Display tab, choose only Backordered
4. Hit OK
