Hi Chaithanya,
If you want make fields mandatory based on the other field value then you need to code in three place, do_finish_input ,save of overview page class and window's method IF_BSP_WD_EVENT_HANDLER~HANDLE_EVENT.
There are following reason why do we need to code in three place.
1. Do_finesh_input because when user select the status value then page will load again this is method called.
2. Save -> There it should not allowed to save the transaction.
3. Window -handle_event method - > when user click on back button so that time system should check is there any validation of mandatory field if so then not allow to go back.
if you need more information then let me know.
Regards,
sunny