A settlement or final invoice is a invoice issued after a proforma invoice at the end of a project, adjusting for actual costs and previous payments to reflect the true value of services provided.
The final invoice has the same issue process as the usual outgoing invoice, just with the difference that you attach the Proforma invoice in the 3rd step of invoicing.
Important note: Allfred allows you to attach this Proforma invoice only if it was marked as paid previously. If it isn't paid, there will be no options to attach.
You can mark the Proforma invoice as paid in Proforma invoice section, using Action menu.