This confirmation window is displayed when you press End Period to to do the end-of-period processing.

There is a 'Finalise completed invoices' check box. You should normally leave this ticked. However, if your practice has a policy that all invoices should be manually finalised by the accounts department, then you should untick this. In this case the completed/not finalised ones will be held over and not included in the closing balance.

Click OK to proceed, else Cancel.