Mr. Greg, after White plays 3. Bxg8, Black will immediately recapture the Bishop with the rook (Black also had the option to play 2...Bxc4). In that case, Black will only have to reveal the placement of the Queen's rook. He need not place a knight or a bishop on b8 indicating the captured piece. He can decide later in the game about the minor piece to be given up by making a move from the other pair.
Mr. Greg, after White plays 3. Bxg8, Black will immediately recapture the Bishop with the rook (Black also had the option to play 2...Bxc4). In that case, Black will only have to reveal the placement of the Queen's rook. He need not place a knight or a bishop on b8 indicating the captured piece. He can decide later in the game about the minor piece to be given up by making a move from the other pair.
I hope I've answered your doubts. Thank you.