Required assistance in ALV filter

Hi Experts,

I am not very good in ALV's I have an issue with REUSE_ALV_GRID_DISPLAY function module.The issue is on the initial output if I select the 1st field BOM Header and click on filter it is working fine.I am getting a selection screen and if I press F4 on select -low I am getting the values.But if I choose another field Recepie do the same exercise I could see only 00000 when I clicked on F4 for the selection screen.For information the BOM Header is char30 and Receipe is N 30 .In the below code I am unable to understand the purpose of passing i_default,I_save and I_variant.Can some tell me the root cause for this issue.Thanks in advance.

Call ALV grid display.

CALL FUNCTION 'REUSE_ALV_GRID_DISPLAY'

EXPORTING

i_callback_program = sy-repid

is_layout = i_layout

it_fieldcat = g_fieldcat[]

it_sort = gi_sort[]

i_default = 'X'

i_save = 'A'

is_variant = i_variant

TABLES

t_outtab = i_final_list

EXCEPTIONS

program_error = 1

OTHERS = 2.

IF sy-subrc <> 0.

MESSAGE ID sy-msgid TYPE sy-msgty NUMBER sy-msgno

WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4.

ENDIF.