The planning period is the number of days allocated by MRP for assessing net demand and is determined by the item’s replenish time.   

Think of the planning period as the item’s “action window.”  When net demand causes an item’s required date to fall within its planning period, a job or PO must be generated to replenish stock by the required date.  When a required date lies beyond the planning period, however, no action is needed because there is ample time to wait and generate a job or PO later when the required date eventually falls into the action window.        

Taking premature action against future requirements is problematic because additional demand can materialize day by day as the required date draws closer to the action window. Generating a job or PO prematurely reacts to an incomplete demand profile and corrupts the “just in time” principle that keeps inventory and WIP as lean as possible. 

If you expect a lower-level job or PO to be generated and it does not occur within the current MRP session, do not worry and do not intervene with any manual action.  The job or PO will be generated in a future MRP session without fail and has no possibility of being forgotten and missed.