The Prepare Time Billing Invoice window has two views:

The Work In Progress view allows you to create time billing invoices for your customers. Using this window to create time billing invoices allows you to review activity slip information for chargeable activities, as well as enter adjustments and create invoices for partial billings.

The Completed Work view allows you to review information about activity slips for which you've billed customers in full. This view also displays information about activity slips you've adjusted so they have $0.00 sums or 0 units left to bill.