When entering sales documents in GP with Manufacturing installed and enabled, you can set a "due date offset" that moves the in-house due date (Table IS010001 / Column SOITEMDUEDATE_I) for the sales line item earlier by the number of days specified. The "Check down days" option also evaluates the in-house due date (and the requested ship date of the sales line item) to see if it is a down day (Table DD010000) and then can either move the date earlier or later (Offset amount and down days preferences stored in ISSP0201). This works in conjunction with Manufacturing's MRP functionality to provide a certain amount of buffer time for scheduling the due dates for purchase orders and manufacturing orders as the MRP calculations use the in-house due date instead of the line item's requested ship date.
As it is now, entering a sales document in SalesPad Desktop results in the in-house due date being set to the same date as the requested ship date of the sales line item. When MRP runs, it then sets the due date for POs and MOs to the same date as the sales line's requested ship date eliminating any buffer.