AT SELECTION-SCREEN 

The event AT SELECTION-SCREEN is the basic form of a whole series of events that occur while the selection screen is being processed.

The standard selection screen, either in an executable program or in the logical database linked to it, is automatically called between the INITIALIZATION and START-OF-SELECTIONevents. When you call the selection screen, and when users interact with it, the ABAP runtime environment generates selection screen events, which occur between INITIALIZATION and START-OF-SELECTION.

You can define event blocks for these events in your program to change the selection screen or process user input.

For further information about selection screen events, refer to Processing Selection Screens.