Get the Sum of Quantity Backordered and Quantity Committed in the Item record

The Multi Location Inventory should be disabled for the formula to work. This is found under Setup > Company > Enable features > Items and Inventory tab.

1. Navigate to Customization > Lists, Records, & Fields > Item Fields.
2. Enter a Name.
3. Set Store Value to False.
4. Under the Applies To tab, select Inventory.
5. Select Type as Free Form Text.
6. Under the Display tab, set Subtab to Purchasing/Inventory.
7. Under the Validation & Defaulting tab, set Formula to True.
8. Set the Default Value to NVL({quantitybackordered},0)+NVL({quantitycommitted},0).
9. Click Save.