Regarding Exit

Hi All

I am new to exits . So need some inputs from all of you.

I need to add functionality to standard CAT2 where it should execute cats_da as soon as a user logs in and in case of any missing time it should popup a warning message to the user to fill that missing time before proceeding for current weeks time sheet filling.

Now am not sure whether this calls for a screen exit or a menu exit or any other approach? any suggestions on the same?

pls guide me on this

Regards

Babita