Due date calculation on Sales Order is based on Document date. The tooltip specifies that Doc. date as the date when the related document was created. Business Central allows shipment and invoice posting from the Sales Order. So user can update the posting date on the service order.

e.g. Sales Order is created on 01 Oct. and shipment and invoice is posted 10 days later. Vendor payment terms are 7 days. System calculates due date as 09 Oct. (01 Oct. + 7D).

So invoice is due even before it is posted. We require a choice on payment terms whether company wants to set payment terms based on document date or on invoice date.