Hi Anuska-
Here is how you can use CHANGING .
Example -
TYPES : BEGIN OF t_itab,
value TYPE char20,
END OF t_itab.
DATA : itab TYPE STANDARD TABLE OF t_itab,
w_itab TYPE t_itab.
w_itab-value = '0001'.
PERFORM fill_itab USING w_itab
CHANGING itab.
FORM fill_itab USING wa TYPE any
CHANGING ptab TYPE INDEX TABLE.
APPEND wa TO ptab.
ENDFORM.