DYNAMIC selection screen ISSUE

Dear Experts,

I have gone through some existing code and found some doubtful line.I have gone through the internet for some document but not able to clear the doubt.

SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME TITLE text-001.

PARAMETERS: r_local RADIOBUTTON GROUP rad1 USER-COMMAND flg, "Local
  r_server RADIOBUTTON GROUP rad1 DEFAULT 'X'. "Server

* Begin - Block 2
SELECTION-SCREEN BEGIN OF BLOCK b2 WITH FRAME TITLE text-002.
PARAMETER: p_local LIKE rlgrap-filename DEFAULT 'C:\' OBLIGATORY.
PARAMETER: p_server LIKE rlgrap-filename DEFAULT '/default' OBLIGATORY.
* End - Block 2
SELECTION-SCREEN END OF BLOCK b2.

* Begin - Block 3
SELECTION-SCREEN BEGIN OF BLOCK b3 WITH FRAME TITLE text-003.
SELECT-OPTIONS: s_kunnr FOR kna1-kunnr, "Customer Number 1
  s_land1 FOR kna1-land1. "Country Key
* End - Block 3
SELECTION-SCREEN END OF BLOCK b3.

* End - Block 1
SELECTION-SCREEN END OF BLOCK b1.

************************************************************************
* At Selection Screen OUTPUT *
************************************************************************
AT SELECTION-SCREEN OUTPUT.

* Check what radiobutton is selected
* Local
  IF r_local = 'X'.
  LOOP AT SCREEN.
  IF "screen-name = 'P_LOCAL'. " OR
  screen-name = '%_P_LOCAL_%_APP_%-TEXT'.
  screen-invisible = '0'.
  screen-input = '1'.
  MODIFY SCREEN.

  ELSEIF screen-name = 'P_SERVER'. " OR
  screen-name = '%_P_SERVER_%_APP_%-TEXT'.
  screen-invisible = '1'.
  screen-input = '0'.
  MODIFY SCREEN.
  ENDIF.
  ENDLOOP.

* Server
  ELSE.

  LOOP AT SCREEN.
  IF screen-name = 'P_SERVER' OR
  screen-name = '%_P_SERVER_%_APP_%-TEXT'.

  screen-invisible = '0'.
  screen-input = '0'.
  MODIFY SCREEN.

  ELSEIF screen-name = 'P_LOCAL' OR
  screen-name = '%_P_LOCAL_%_APP_%-TEXT' OR
  screen-name = '%_S_KUNNR_%_APP_%-TEXT' OR
  screen-name = '%_S_KUNNR_%_APP_%-OPTI_PUSH' OR
  screen-name = 'S_KUNNR-LOW' OR
  screen-name = '%_S_KUNNR_%_APP_%-TO_TEXT' OR
  screen-name = 'S_KUNNR-HIGH' OR
  screen-name = '%_S_KUNNR_%_APP_%-VALU_PUSH'. .

  screen-invisible = '1'.
  screen-input = '0'.
  MODIFY SCREEN.
  ENDIF.
  ENDLOOP.
  ENDIF.

My doubt is why use use the below statement ? means what is the exact use of the statement and in which case we generally use this statement ?

'%_P_LOCAL_%_APP_%-TEXT'

'%_S_KUNNR_%_APP_%-TEXT'

how the system comes to know about the statement ?

Please clear my doubt and if posible then send me some document.

Kind Regards,

Ajit