Tx_ofdm doubts

hey Marcus,
Thanks for all your help, now we are able to create our own custom block
in
GNU radio.
While working on given example tx_ofdm.grc, we have some conceptual
doubt
regarding chunk to symbol block…

  1. Could you explain me what exactly Symbol table means and how does it
    function and also its significance, and what does payloads_mode()
    mean…?
    2)and for packet header generator what does Formatter object mean…?

screen shot is shown in the attachment
tx.png for 1st
and payload_mod.points().png for 2nd