Greetings Adithya,
You might need to go into development for this. I can think of a few solutinons within the standard, but they're quite a bit clunky.
You can't define a standard Field Selection for the Notification to be influenced by Order Assignment, therefore you have no direct connection and you must look for a work-around or development.
If you want to develop a solution, then you will might need to modify the screens for SAPLIQS0 and make the editability of the fields dependent on the status ORAS, or on a user status -
Or just prevent saving a Notification with status ORAS if changed in the transaction, with user exit QQMA0014 (unless it is being completed)
You could create three User statuses e.g.
OSNO (initial)
ORAS (set automatically when the Order is assigned)
CHNG (change mode)
Last status could be a provision to change the Notification if required, use an authorisation key for the last order to only allow the Notification requestor role to allow changes.
Thyagarajan Krishnamurthy, I don't know of a business transaction that would allow you to forbid any Change altogether to the Notification based off a User status?
Also, I can think of 3 options within the standard configuration:
1) Define a Notification Type that is a copy of your current type, but for Notifications with Order assigned. Define Field Selection influenced by Notification Type and define Allowed Notification Type Change. Then when the Order is created, you could change the type and have the fields locked. Also, you could make authorization restrictions based off the Notification Type. The downside is that this is a manual intervention for the user.
2) Define a Priority e.g. "Assigned". Define Field Selection influenced by that Priority. Then when the Order is created, you could change the Priority and have the fields locked down. The downside is that this is a manual intervention for the user, you can't prevent the user from changing the Priority back and then making further changes, and this will conflict with date calculation based on Priorty when it is changed, if you set that up in your Priorty Types.
3) Simply have the Notification completed at once when you assign the Order - with the obvious downside that you can't update it or see it in standard report selections as "In process". You can however update if you take the Notification "Back to processing" when required, so you could work around that with authorizations and status profiles.