START-OF-SELECTION
This event occurs after the selection screen has been processed and before data is read using the logical database. You can use it to prepare for reading data and creating the list by, for example, setting values for internal fields and writing introductory notes on the output list.
In an executable program, any non-declarative statements that occur between the REPORT or PROGRAM statement and the first processing block are also processed in the START-OF-SELECTION block. See also
Defining Event Blocks. 
The following program is connected to the logical database F1S.
REPORT EVENT_DEMO.
NODES SPFLI.
...
AT SELECTION-SCREEN.
...
START-OF-SELECTION.
WRITE: / 'List of Flights' COLOR COL_HEADING,
/ 'Created by', SY-USERID, 'on', SY-DATUM.
ULINE.
GET SPFLI.
...
