What is the option to "Include projects without unbilled data" on the Billing - Add Invoice screen?
This option allows you to create a $0 invoice to which you may then add custom Additional Items. The term "unbilled" means "Posted." The typical process in Unanet is to Post and then Add Invoice. If there is nothing to Post (fixed price items, labor, expense, etc.), the project is "without unbilled data." Checking this box allows you to proceed with creating an invoice even though there was nothing to Post.