The letter "X" shown to a hand's right indicates that the hand is an exposed hand, meaning you may call a tile and expose it to complete a pung, kong, quint, or sextet
If a player picks a tile from the wall, declares mah jongg, and starts to expose his tiles only to find out he did not have mah jongg, their hand would be considered dead.
A player may not use a Joker for either a single tile or in a group of single tiles. This rule also applies to pairs or a group of tiles with pairs. A single tile defined is a stand-alone tile or a group of tiles that are not identical i.e. 2020 or NEWS. During the game, a player may not call to expose a group of single tiles unless the tile called is the winning tile that gives them mah jongg.