GRC Notification subject Line can be customized as per need attaching an example.
Go to Tcode SE16
Look for Table GRFNVNOTIFYMSG
This table contains the information about notification messages
Such as Message Class
Now select(double Click) the Message class for which you want to change subject line.
And Change the subject line in below highlighted area.
I have added Request number and User id with little details about action.
You can also customize based on your need.