The Add New Invoice screen is for adding invoices manually. Just like you can generate Invoices through turning in assets, you can do the same manually from this screen.
Fill in the required fields indicated by the * on the screen. Also add any additional info in the Description if necessary and assign the invoice to a certain Task number if the invoice is associated with a task. The "Date" fields will pop up a calendar when you click in those fields for you to input the date.
After adding all the information for the invoice, click the Save button. This will automatically generate an Invoice ID and create the invoice. The screen will turn from an Add New Invoice screen to an Edit Invoice screen. From here you can delete the invoice, make any changes, print the invoice, or add any payments.
If you would like to see an example of how to add an invoice, click Here