BDC: Diffrence between Format_message and Write_message

hi abapers can u please explain format_message and write_message.

if i use call transaction syntax like

call transaction 'mk01' using bdctab mode 'N' update 's' messages into bdc_msg.

shall i take format_message or write_message and wht parametrs we should pass in that.

if i put mode 'E' error screen mode wht should i do.

please guide me.