Use Of At Selection-screen Output

REPORT  znnr_report NO STANDARD PAGE HEADING MESSAGE-ID znnr LINE-SIZE 100 LINE-COUNT 65(4).
******DATA DECLARATIONS**********
DATA : BEGIN OF it_plant OCCURS 0,
        matnr LIKE mara-matnr,
        werks LIKE marc-werks,
        pstat LIKE marc-pstat,
        ekgrp LIKE marc-ekgrp,
       END OF it_plant.
DATA : BEGIN OF it_pono OCCURS 0,
        ebeln LIKE ekko-ebeln,
        ebelp LIKE ekpo-ebelp,
        matnr LIKE ekpo-matnr,
        werks LIKE ekpo-werks,
        lgort LIKE ekpo-lgort,
       END OF it_pono.
TABLES ekko.
********END OF DATA DECLARATIONS*********
*******SELECTION SCREEN DESIGN **********
SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME TITLE text-001.
PARAMETER : p_werks LIKE marc-werks MODIF ID s1.
SELECT-OPTIONS : s_ebeln FOR ekko-ebeln NO INTERVALS MODIF ID s2.
SELECTION-SCREEN END OF BLOCK b1.
SELECTION-SCREEN BEGIN OF BLOCK b2 WITH FRAME TITLE text-004.
SELECTION-SCREEN BEGIN OF LINE.
PARAMETERS : r1 RADIOBUTTON GROUP g1 DEFAULT 'X'.
SELECTION-SCREEN COMMENT 5(20) text-002 FOR FIELD r1.
SELECTION-SCREEN END OF LINE.
SELECTION-SCREEN BEGIN OF LINE.
PARAMETERS : r2 RADIOBUTTON GROUP g1.
SELECTION-SCREEN COMMENT 5(20) text-003 FOR FIELD r2.
SELECTION-SCREEN END OF LINE.
SELECTION-SCREEN END OF BLOCK b2.
******END OF SELECTION SCREEN DESIGN****************
*********INITIALIZATION OF SELECTION SCREEN ELEMENTS.*****
INITIALIZATION.
  p_werks = '1000'.
  s_ebeln-low = '4500016926'.
  s_ebeln-option = 'EQ'.
  s_ebeln-sign = 'I'.
  APPEND s_ebeln.
  CLEAR s_ebeln.
************END OF INITIALIZATION***********************
***********SCREEN MODIFICATIONS*******************
AT SELECTION-SCREEN OUTPUT.
  LOOP AT SCREEN.
    IF r1 EQ 'X' AND screen-group1 EQ 'S2'.
      screen-input = 0.
      MODIFY SCREEN.
    ENDIF.
    IF r2 EQ 'X' AND screen-group1 EQ 'S1'.
      screen-input = 0.
      MODIFY SCREEN.
    ENDIF.
  ENDLOOP.
********END OF SCREEN MODIFICATIONS*****************
**************SCREEN VALIDATIONS ****************
AT SELECTION-SCREEN.
  SELECT SINGLE *
           FROM ekko
           INTO ekko
           WHERE ebeln IN s_ebeln.
  IF sy-subrc <> 0.
    SET CURSOR FIELD 'S_EBELN-LOW'.
    MESSAGE e999 WITH text-005.
  ENDIF.
********end of screen validation*****************
START-OF-SELECTION.
*set pf-status '100'.
  IF r1 EQ 'X'.
    SELECT matnr
           werks
           pstat
           ekgrp
       FROM marc
       INTO TABLE it_plant
       WHERE werks = p_werks.
    LOOP AT it_plant.
      WRITE : sy-vline , 2 it_plant-matnr COLOR COL_KEY,
             21 sy-vline , 22  it_plant-werks COLOR COL_KEY,
             27 sy-vline ,28 it_plant-pstat COLOR COL_NORMAL,
             43 sy-vline ,44 it_plant-ekgrp COLOR COL_NORMAL.
    ENDLOOP.
  ENDIF.
  IF r2 EQ 'X'.
    SELECT ebeln ebelp matnr werks lgort
           FROM ekpo
           INTO TABLE it_pono
           WHERE ebeln IN s_ebeln.
    LOOP AT it_pono.
      WRITE : sy-vline , 2 it_pono-ebeln COLOR COL_KEY,
             12 sy-vline , 13 it_pono-ebelp COLOR COL_KEY,
             18 sy-vline , 19 it_pono-matnr COLOR COL_NORMAL,
             37 sy-vline , 38 it_pono-werks COLOR COL_NORMAL,
             44 sy-vline , 45 it_pono-lgort COLOR COL_NORMAL, 49 sy-vline..
    ENDLOOP.
  ENDIF.

TOP-OF-PAGE.
  IF r1 EQ 'X'.
*ULINE AT /1(48).
    WRITE : sy-vline ,2 'MATERIAL NUMBER',
            21 sy-vline , 22 'PLANT',
            27 sy-vline , 28 'STATUS',
            43 sy-vline , 44 'GRUP', 48 sy-vline.
    ULINE AT /1(48).
  ENDIF.
  IF r2 EQ 'X'.
    WRITE : sy-vline , 2 'PO NUMBER',
           12 sy-vline, 13 'ITEM',
           18 sy-vline,19 'MATERIAL NUMBER',
           37 sy-vline, 38 'PLANT',
           44 sy-vline, 45 'GRUP',
           49 sy-vline.
    ULINE AT /1(50).
  ENDIF.

END-OF-PAGE.
  ULINE AT /1(50).
  WRITE :/10 'PAGE NUMBER', sy-pagno.