Hi Andrew, 

Invoice Due Date basis is the date X3 uses to calculate the Due Date of Invoice in conjunction with the payment terms specified. Due Date is the date the item is due for payment by the customer.

For example lets assume I have created an invoice for a customer whose payment terms are invoice date + 45 days.

Creating a new Customer BP invoice on 09/30/21 would default the Due Date Basis to the same date. Taking into account the payment terms specified on the invoice header (which defaults from the Customer record), the Due date of the invoice would be 11/14/21.

In your example, no payment terms have been specified on the invoice header and so the Due Date is the same as the Due Date Basis, i.e. payable immediately.

Best regards,

Nick