When do you void and when do you delete an existing invoice or credit memo that you don't want anymore? This depends on whether or not you need to maintain an audit trail for the transaction. In some cases an audit trail is not needed—for example, if you create an invoice or credit memo and, without ever printing it, realize that it is in error.
Other times you will need to keep a record of the invoice or memo, and then you will want to void rather than simply delete it. Most commonly, you will want to void an invoice or credit memo if it failed to print on your preprinted form because of a paper jam. For instance, say the printer jammed while printing Invoice No. 101. The invoice register will have a record of Invoices No. 100 and 102 but not of 101 because it failed to print. In this case, to maintain a proper audit trail and account for the missing invoice, you would void rather than delete Invoice No. 101.