It's the Planned Due Date on the customer order line that is used. This represents the date when the material needs to be available in stock to be shipped and delivered in time. 

@Björn Hultgren thanks, and am i correct in thinking that the due date takes into consideration the exception codes set on the manufacturing and distribution calendar. i.e. Christmas holiday

regards

Jamie 

MRP works internally with the manufacturing calendar only. So let say that you for example have a planned due date on the customer order line 24th December - and that is also a working day in your Distribution calendar, but not in the Manufacturing calendar. When MRP does it snaphot of open supply and demand orders it will move this demand date to the 23rd December (if 23 is a manufacturing calendar day). So in MRP Part Information you will see the 23rd December of that CO Line Demand, but if you navigate to IPAP you will see the 24th December.

So you are correct Jamie

Happy Xmas

-Mats