The heading portion of this window displays the Undistributed balance and the Subject to retention (a running balance of the 'Amt this period' for rows where the Retention checkbox is selected) amounts.

While it is possible to have multiple un-updated billings at the same time, you must adjust job-to-date figures of other un-updated billings when an editing quantities or amounts of the earlier billing.

When selected, the Show All Billing Items button lets you recompile the complete schedule of values, thus allowing changes. When the invoice is saved, only the non-zero records will be moved along. You can click this button and recompile the list again up until the invoice is posted.