AT SELECTION-SCREEN event of report in sap abap

When user enters the values in the fields of selection screen and clicks on execute button, this event gets triggered. This event is basically for checking the values entered by the user for the fields of the selection screen i.e., data validity checking. This event is for entire selection screen. For example:

You are accepting carrid, connid, fldate from user and you don’t want to proceed if user enters no value for carrid and fldate. Using AT SELECTION-SCREEN can do this.

Select-options: carrid1 for sflight-carrid,
Connid1 for sflight-connid,
F1date1 for sflight-f1date.

AT SELECTION-SCREEN.
If carrid1-low ne ‘ ’ and fldate1-low = ‘ ’.
Error message.
Endif.

In this case, if both the fields are entered blank, then the user gets error message.
Basically, this event is for many fields on selection screen. Usually, it is for the fields which are logically related.