REG: Sales order changes

Hi all,

I have a scenario where ,

There is a delivery block to the sale order satisfying some conditions.

I could implement it in MV45AFZZ.

Now there is one more problem,

For the same Order which has delivery block

if the user wants to change the Delivery Block, then the exit must trigger.(Its while changing the Sales Order)

can someone tell me under which routine should i need to code in MV45AFZZ.

Can anyone give clue about the same logic.

Thanks and Regards

Srikanth.P