Wikipedia
X-FEN
X-FEN is an extension of Forsyth-Edwards Notation (FEN).
The traditional Forsyth-Edwards Notation is not sufficient to represent all possible positions in 8x8 Chess960 (aka Fischer Random Chess) or 10x8 Capablanca random chess (CRC). Consequently, an extension of FEN was needed, with the requirement of being fully backward compatible. X-FEN (formerly FRC-FEN), introduced by Reinhard Scharnagl in 2003, accomplishes this.