Reference to an invoice to link the credit note to. The link is reference-only: the invoice's line items are not copied, and the invoice is not cancelled. Three things do depend on the link — the exchange rate is taken from the invoice when the company setting for it is enabled and none is sent; the cash-discount terms keep their regular defaults instead of being zeroed as they are on a credit note without a link; and with the company setting "Dunning with account reconciliation" enabled, every save of the credit note reopens the invoice (paid amount reset, payment status open), unless the invoice is paid in full, write-protected or has its dunning status fixed.
