I have a sales order within PP/DS horizon.
And a firmed planned order outside PP/DS horizon.
Why is the MRP heuristic considering a planned order outside the PPDS horizon, to cover the requirement within the PP/DS Planning Horizon?
The net requirement calculation is the phase responsible for calculating the product shortage on MRP and some other planning heuristic.
And, the net requirements calculation considers the entire horizon. Not only PP/DS planning horizon.
Check the following SAP notes:
448960 - Net requirements calculation (documentation)
2474628 - Net Requirement Calculation common issues
There it is said:
"10) How is the PP/DS horizon taken into account in the net requirements calculation?
The PP/DS horizon is only used to check whether SNP receipt elements participate in the net requirements calculation (see above). The net requirements calculation is always carried out over the complete time axis."
Change the net requirements procedure to 'Avoid delays', and the delayed planned orders, even firmed, will not be used in net requirements calculation. A new planned order will be generated.
With this in mind, the solution for this kind of scenario is:
1) change the procedure to avoid delays, or
2) review why there are delayed and firmed orders outside horizon, or
3) use a scheduling heuristic to bring the orders to a date closer to the requirement date. You may use the top down heuristic for example.