Let me share some insights about the Pending Build reports in QuickBooks Desktop (QBDT).
When creating build assembly, you need to enter a quantity in the Quantity To Build field and then, you have an option to make it as pending. This will reflect on the Pending Build report which represents as "To be Build Qty".
For additional reference, you can pull up the Pending Build report and click the Help option at the top menu bar in your QuickBooks. It will show you the documented information about the Build Pending feature.