WRITE_FIELD_SEPARATOR is a flag whether fields shall be separated and not the separating character, see function module documentation:
Separate Columns by Tabs in Case of ASCII Download
Description
In the downloaded file, the columns are separated by tab characters (cl_abap_char_utilities=>horizontal_tab). You should use this setting if you want to upload the data from the file at a later time, because this is the only way of identifying individual columns.
The parameter makes sense only for the FILETYPE values ASC, DAT and IBM; for DAT it is set implicitly.
Value range
'X' : Write separator.
SPACE : Do not write separator.
Default
SPACE
