아래 소스중에 매크로 내용이 있는데 어떻게 연결이 되어 있나요?

'VKORG' 'VBAK'. 이 두개가 fieldname과 ref_tabname인가요?

ls_fieldcat-col_pos은 어디와 연결이 되는지 이해도 않가고..

아래소스를 보시고 자세한 답변부탁합니다.

* Macro definition
DEFINE m_fieldcat.
  add 1 to ls_fieldcat-col_pos.
  ls_fieldcat-fieldname   = &1.
  ls_fieldcat-ref_tabname = &2.
  append ls_fieldcat to lt_fieldcat.
END-OF-DEFINITION.

TABLES : vbak.                         " Sales Document: Header Data

TYPE-POOLS: slis.                      " ALV Global types

SELECT-OPTIONS :
  s_vkorg FOR vbak-vkorg,              " Sales organization
  s_kunnr FOR vbak-kunnr,              " Sold-to party
  s_vbeln FOR vbak-vbeln.              " Sales document

SELECTION-SCREEN :
  SKIP, BEGIN OF LINE,COMMENT 5(27) v_1 FOR FIELD p_max.    "#EC NEEDED
PARAMETERS p_max(2) TYPE n DEFAULT '20' OBLIGATORY.
SELECTION-SCREEN END OF LINE.

DATA:
* Data displayed in the first list
  BEGIN OF gt_vbak OCCURS 0,
    vkorg LIKE vbak-vkorg,             " Sales organization
    kunnr LIKE vbak-kunnr,             " Sold-to party
    vbeln LIKE vbak-vbeln,             " Sales document
    netwr LIKE vbak-netwr,             " Net Value of the Sales Order
  END OF gt_vbak,

* Data displayed in the popup list
  BEGIN OF gt_vbap OCCURS 0,
    posnr  LIKE vbap-posnr,            " Sales document item
    matnr  LIKE vbap-matnr,            " Material number
    arktx  LIKE vbap-arktx,            " Short text for sales order item
    kwmeng LIKE vbap-kwmeng,           " Order quantity
    netwr  LIKE vbap-netwr,            " Net value of the order item
  END OF gt_vbap.

*---------------------------------------------------------------------*
INITIALIZATION.

  v_1 = 'Maximum of records to read'.

*---------------------------------------------------------------------*
START-OF-SELECTION.

  PERFORM f_read_data_vbak.

  PERFORM f_display_data_vbak.

*---------------------------------------------------------------------*
*      Form  f_read_data_vbak
*---------------------------------------------------------------------*
FORM f_read_data_vbak.

  SELECT * INTO CORRESPONDING FIELDS OF TABLE gt_vbak
           FROM vbak
             UP TO p_max ROWS
          WHERE kunnr IN s_kunnr
            AND vbeln IN s_vbeln
            AND vkorg IN s_vkorg.

ENDFORM.                               " F_READ_DATA_VBAK
*---------------------------------------------------------------------*
*      Form  f_display_data_vbak
*---------------------------------------------------------------------*
FORM f_display_data_vbak.

  DATA:
    ls_fieldcat TYPE slis_fieldcat_alv,
    lt_fieldcat TYPE slis_t_fieldcat_alv.

* Build the field catalog
  m_fieldcat 'VKORG' 'VBAK'.
  m_fieldcat 'KUNNR' 'VBAK'.
  m_fieldcat 'VBELN' 'VBAK'.
  m_fieldcat 'NETWR' 'VBAK'.

* Display the first list
  CALL FUNCTION 'REUSE_ALV_GRID_DISPLAY'
       EXPORTING
            i_callback_program      = sy-cprog
            i_callback_user_command = 'USER_COMMAND'
            it_fieldcat             = lt_fieldcat
       TABLES
            t_outtab                = gt_vbak.

ENDFORM.                               " F_DISPLAY_DATA_VBAK
*---------------------------------------------------------------------*
*       FORM USER_COMMAND                                             *
*---------------------------------------------------------------------*
FORM user_command USING u_ucomm     LIKE sy-ucomm
                        us_selfield TYPE slis_selfield.     "#EC CALLED

  CASE u_ucomm.
    WHEN '&IC1'.
      READ TABLE gt_vbak INDEX us_selfield-tabindex.
      CHECK sy-subrc EQ 0.
      PERFORM f_read_data_vbap.        " Read data from VBAP
      PERFORM f_display_data_vbap.
  ENDCASE.

ENDFORM.                               " USER_COMMAND
*---------------------------------------------------------------------*
*      Form  f_read_data_vbap
*---------------------------------------------------------------------*
FORM f_read_data_vbap.

  SELECT * INTO CORRESPONDING FIELDS OF TABLE gt_vbap
           FROM vbap
          WHERE vbeln = gt_vbak-vbeln.

ENDFORM.                               " F_READ_DATA_VBAP
*---------------------------------------------------------------------*
*      Form  f_display_data_vbap
*---------------------------------------------------------------------*
FORM f_display_data_vbap.

  DATA:
    ls_private  TYPE slis_data_caller_exit,
    ls_fieldcat TYPE slis_fieldcat_alv,
    lt_fieldcat TYPE slis_t_fieldcat_alv.

* Build the field catalog
  m_fieldcat 'POSNR'  'VBAP'.
  m_fieldcat 'MATNR'  'VBAP'.
  m_fieldcat 'ARKTX'  'VBAP'.
  m_fieldcat 'KWMENG' 'VBAP'.
  m_fieldcat 'NETWR'  'VBAP'.

  ls_private-columnopt = 'X'.          " Optimize width

* Display items in a POPUP
  CALL FUNCTION 'REUSE_ALV_POPUP_TO_SELECT'
       EXPORTING
            i_selection = ' '
            i_tabname   = 'GT_VBAP'
            it_fieldcat = lt_fieldcat
            is_private  = ls_private
       TABLES
            t_outtab    = gt_vbap.

ENDFORM.                               " F_DISPLAY_DATA_VBAP
**************** END OF PROGRAM Z_ALV_GRID_AND_POPUP ******************