What is the use of COL_SELECT_MASK IN fm gui_download?

Hi i Just want to know what is the use of col_select_mask in GUI_DOWNLOAD.

currently i need to work on one report where they are passing this parameter to gui_download.

result is it is taking only 255 char column length in the downloaded excel...

i need to add extra fields to the downloaded excel..but it is taking only 255 how to over come this issue?

i tried commented the COL_SELECT_MASK but it is taking unnecessary fields in the internal table i dont know how come they are excluding some fields without downloading ?Any one any idea?

Regards

sas .