Why should I authorize my MTF trade with CDSL and does it involve any charges?

As per SEBI guidelines, it is mandatory to authorize MTF Positions on the CDSL website. 
This is because when a broker provides funding through MTF, they need to pledge the MTF  shares with CDSL. Pledging is done to protect the interest of the broker, in case a client is unable to repay the borrowed amount. 

For this, CDSL charges a one-time fee of Rs.20(+tax) and when you exit the MTF position the shares are unpledged and CDSL will charge Rs.20 (+tax) again