Hi KH Fong,
Refer the below explanations one by one.
Spool is being controlled by BASIS team and you can decide the retention period. So ask your technical person to increase the retention period more if you want. Transaction is SPAD. But it should not be like 100,200.... days. It impacts the performance.
Once MRP has deleted the planned order, you can not see it in PLAF table. The table doesn't keep deleted planned order. This is true once you are converting the planned order to prod/process order.
Planning mode 3 means system will delete and recreate the MRP element(planned order/prod order/PR etc) once again. Only the in process elements are not getting deleted. This is useful if your MRP run has happened with wrong data where you want to run with correct data again or at the beginning of the month where the unconverted planned order has to be deleted.
All depends upon the planning scenario used by the plant.
Check the note as mentioned by Rajen for more details.
Regards,
Krishnendu.