Why do we need to setup replinishment type to consider PO supply in min max planning

Summary: PO supply was not being considered in min max planning even after giving the supply cut off date. Oracle suggessted to update in product management under specifications--> Planning--> Sourcing--> replenishment type to Subinventory. Only after doing this was the PO supply visible

1. Why is this setup not mentioned in the oracle documentation: https://docs.oracle.com/en/cloud/saas/supply-chain-and-manufacturing/23d/famml/min-max-planning.html#s20031253
2. Further more this setup is already done in manage subinventory and locators why do we need to do the same in product management only then PO supply is visible
3. Also PO supply is seen only if the supply cut off date given is greater than the need by date of the PO. If we leave this field blank is program is considering it as sysdate and not seeing the Open PO's . Can this be defaulted instead of sysdate.