Hello Mr. Canan.
After I reread your comment, I finally find the way to add the custom MRP element. As you said, that the value was added from SE11, add value range of DELKZ. That's really helpfull.
Now, we go to the second "mystery" of my SAP R3 system. Originally, SAP system will create MRP element "LA" (ShpgNt / Shipping Notification) when an Order Acknowledgment (in ME23N) was created.
However, my SAP R3 system automatically assign Order Acknowledgment into that custom MRP element "99". Do you have any clue, where will I find the EXIT / BADI for that matter?
Thank you a lot.