What is the difference between a prevalent wind and a seat wind?

The prevalent wind (圈風) is the wind for the current round. During an East round, East is the prevalent wind for all players. The seat wind (門風) is specific to each player's individual seat position. A pung of the prevalent wind scores 2 fans for any player at the table, while a pung of your seat wind scores 2 fans only for the player sitting in that seat. If your seat wind matches the prevalent wind (e.g., you are East in an East round), a pung of that tile scores both bonuses simultaneously.