The unique identifier for the player within the hand. This Id is used to identify the player in all other locations of the hand history. This player identifier is hand history specific and does not need to be the same across hand histories.
The player "id" is simliar to the "seat"; however, it should be independent from the seat for the case when a player gets up and a new player is seated in the middle of a hand. In this case, the players would have different player ids but the same seat number.