Requirement for an issue is covered by a planned receipt for the period set in the positive days .Positive days are known as shelf life of an item.
As you see in the manual , it is clearly defined as " Should master scheduling generate a new planned purchase order to cover the requirement, or do you accept that the order is fulfilled from the current inventory? ".This question is determined by Positive days.
And one more note from the manual ;"If you use automatic reservation when you create sales orders, or manual reservation later, Positive days are not considered in master scheduling. "
Best Regards,
Nkeskin