Forum:Pieces of eight

I'm still confused about what makes a Pirate Lord's Piece of eight special. It says they were created to bind Calypso, but the pieces are unique to the CURRENT pirate lord and not the one that was present when Calyso was bound. So it seems that a pirate lord can somehow choose his piece when he becomes a pirate lord.

In that case, it is questionable why they needed to rescue Jack in part 3 and why elisabeth had used Sao Feng's piece of eight.