AT SELECTION-SCREEN selscreen_event.
This statement defines event blocks for different events selscreen_event that are triggered by the ABAP runtime environment during selection screen processing.
Selection screen events occur immediately before sending a selection screen and after certain user actions on a displayed selection screen. They assist in selection screen processing in the ABAP program.
AT SELECTION-SCREEN is triggered at least twice during actions on selection screens that are linked into another selection screen as a
subscreen - first for the linked selection screen itself, and then for the linking selection screens.
The event blocks after AT SELECTION-SCREEN are implemented internally as
procedures. Declarative statments in these event blocks create local data.