A selection screen is a standardized screen for entering data and selection ranges. It also has a standardized user interface, that you cannot affect to the same extent that you can work with the user interfaces of screens and lists.
Selection screens are used primarily for entering data. When the user selects functions the corresponding function codes from the GUI status are not processed in the ABAP program in which the selection screen is defined. Instead, they are processed directly by the ABAP runtime environment. Some functions are executed by the ABAP runtime environment itself (for example, saving the selection screen contents as a variant, or diverting the list output from the screen to the spool system). Other functions, of which Enter and Execute ( F8 ) are the most important, trigger selection screen processing in the ABAP program. Here, special event blocks are called in the ABAP program. The triggering function from the ABAP program is not normally necessary.
The following sections describe the ways in which you can change the standard process: