AT SELECTION-SCREEN ON field event in reports sap abap

When you want to check for specific value of a field. For example, carrid should be in the range of ‘LH’ and ‘SQ’. This can be done in this event. Basically, this event is for checking individual fields. You can have many AT selection-screen events in your program (i.e., for each field specified in the Select-Options).

Select-Options carrid1 for sflight-carrid.
AT SELECTION-SCREEN.
If carrid1-low ne ‘LH’ and carrid1-high ne ‘SQ’.
Error message.
Endif.
Here the system will not proceed on entering wrong values.