Hi Ajit,
The User-command statement is used to perform some action when the user makes changes to the screen. For example, if I have 2 radio buttons and for the 1st button i have to show 2 fields as selection fields and for the 2nd button I have to show 3 fields then based on the user command selected by the user (either 1st rbadio button or 2nd radio button) logic can be written.
With Regards,
Giriesh M