How ABAP implements screen fields that cannot be entered

FORM LFILE_SFILE_DISPLAY.

  LOOP AT SCREEN.

    IF  SCREEN -NAME = ' P_LFILE ' .
       IF P_KUBUN = CNS_KEY_L.
         SCREEN -INPUT = 1 .   " Enable ELSE .
         SCREEN -INPUT = 0 .   " Can 
      not enter ENDIF .
     ENDIF .
     IF SCREEN -NAME = ' P_SFILE ' .
       IF P_KUBUN = CNS_KEY_S.
         SCREEN -INPUT = 1 .   " Can enter ELSE .
        
       
      SCREEN -INPUT = 0 .   " Can not enter 
      ENDIF .
     ENDIF .
     MODIFY  SCREEN .

  ENDLOOP.

ENDFORM.                    "LFILE_SFILE_DISPLAY