Invoice Number Format

This is a required field when setting up a project's invoice attributes.  The list of available options is defined by the Administrator on the Admin >> Setup >> Invoice Number Formats screen.  

Changing the Invoice Number Format on a project will have no impact to any existing draft or completed invoices and will only affect the layout of future invoices.

Note that invoice numbers are guaranteed to be unique across any projects that may be using a specific format (but will not be unique within a project).  Invoice numbers are not guaranteed to be sequential without gaps (that is, if you create a draft invoice and delete it, upon creating a subsequent invoice the number that appeared on the deleted draft invoice will not be re-used).

Note that you can modify the Invoice Number on an invoice once it has been generated (via the List Invoices >> Edit Setup screens).