Hi Rajesh,
I can't tell you why SAP decided to allow some places to be editable and some others not. All I can tell you is that from a long time it hasn't been possible to change information on the rows of documents like you asked in your original post.
I've seen many post of people asking for it (and been in need of such functionality myself) but as far as I know it's not possible.
You can open a ticket with SAP Support to have a more "official" explanation. Unfortunately this is all I can offer you.
If you find a solution or contact SAP please post the answer so that we all benefit.
Good luck.
Best regards,
Pedro Magueija
