In basic terms, a PreBill invoice is a draft and you can edit it. A Finalized invoice is "locked" and ready for billing. That means if you want to edit a Finalized invoice, you have to un-finalize it. Want to edit a Finalized invoice? Here's how.
With a PreBill invoice, you can click on time and expense entries and edit them directly. All invoices pass through the PreBill stage before you finalize and send them to Clients for payment. You can search for and see all PreBill and Finalized Invoices on the "Invoicing" Tab, "Invoice List" Sub Tab.
Here's an example:
A Finalized Invoice is "locked" and ready for billing. You can always delete a Finalized Invoice, but you can't make edits to entries on it without switching it back to "Pre-Bill" status. You must finalize invoices before you can email them to clients for payment. You do this by clicking the "Finalize" button that appears on the PreBill Invoice. Only finalized invoices (whether they are fully paid, partially paid or open) show up in the "Accounting" Tab of Time and Billing.
On the "Accounting" Tab, "History" Sub Tab, you can see a history of finalized/billed invoices and payments.
See the picture here: