If the setting Preliminary coding of invoices? is activate you will see the field called Prel. coding date. You find this setting under the Accounts Payable heading under the Purchase tab in the Settings procedure.
The setting Suggest same voucher date as invoice date? determines how the preliminary coding date will be set. If the setting is activated the Preliminary coding date will be the same as the invoice date. If the setting is not activated, then the Preliminary coding date will be set as today's date.