Different Function Code for "SAVE" button

Hi all,

I have a requirement to validate thge entries for the PO entered in the screens ME21,ME22,ME23,ME21n,ME22n,ME23n before saving the PO.

I have found the revelant EXIT for the validation.

Now the PO can be saved in different ways and through different screens from the above transactions.For eg from ME21 the user can go into the "Item detail specific screen" and "SAVE" the PO or can hit the "BACK" and press the "YES" button to save in the pop-message. Now this triggers different function codes for the "SAVE".

There might also be different other similar scenarios for saving the PO in different screens.

Can we capture all the function codes for the SAVE button before hand in the EXIT and validate the PO before saving for this function codes?

Is there any table which maintains this information from where it can be retrieved.Any other workaround for this?

Need your ideas on this.

Thanks,

stock