To get the selection screen in module-pool(Sub screen concept)

* Create the subscreen area in the main screen and write the below code in the Top incuude of the program

  TABLES ikpf.
SELECTION-SCREEN BEGIN OF SCREEN 2300 AS SUBSCREEN.
SELECT-OPTIONS: s_bldat for ikpf-bldat .
SELECTION-SCREEN END OF SCREEN 2300.

* And call the subscreen in the PB and PAI module.
  PROCESS BEFORE OUTPUT.
* To call the subscreen into main screen
  CALL SUBSCREEN: G_SUBSCREEN INCLUDING sy-repid '2300'.
 
PROCESS AFTER INPUT.
* To capture the subscreen calues.
 CALL SUBSCREEN g_subscreen.

* Here 2300 -> screen number which u created in the top include.
*     G_Subscreen -> is the subscreen ares name in the mainscreen which we       * gave while creating the subscreen.

*sy-repid -> our program name.