You can change the status of an invoice.
Once the invoice is settled, the total is removed from the Invoiced Balance on the account the invoice is attached to. The same is true in the invoice is voided.
You can generate a PDF version of the invoice which, once compiled, will be attached to the consignment and can be issued to the customer.
Once and invoice has been issued, it cannot be voided. A credit note wil have to be raised if the invoice is has been raised in error.
Credit notes are identical to invoices.
The only difference is that the total for a credit note is negative.