The Race Piece (種のコマ, Shu no Koma) is a chess piece bestowed upon the representative leaders of each race. Imanity owns the King, the Flügel owns the Queen, the Elves owns the Rook, the Werebeasts own the Pawn, and the rest have yet to be revealed. It is later revealed that, by conquering all the Exceed races, and thus having all the 16 chess pieces, one attains the right to challenge Tet for the title of One True God, as a chessboard consists of 16 pieces. Tet did not create the concept of the race pieces, however, Riku and the "179 Ghosts" of humanity did.
The Race Piece holds the rights of the race they belong to, so while the Races Pieces can be bet, it is unwise to do so. If a race's representative bets their race's Piece and loses, the race's rights as an Exceed are stripped and the pledges no longer apply to them. Hence no one has ever bet their Piece in the past, Sora and Shiro being the first ones to do so.
Sora and Shiro was the first representative to bet his race's Piece, and after winning, they was also the first to succeed in holding on to it.