When a player calls a tile and places it on the flat part of their rack or exposes tiles from their rack, they are committed to taking that discard. They may add or take away from the exposure before discarding.
One might think that a player's turn begins when the previous player discards a tile but that is not the case. The reason for this is that turns can be interrupted when another player claims the discard to make an exposure. Therefore, a player's turn officially begins when they either pick a tile from the wall or call a discard.
A commitment to a called tile occurs when a player either exposes tiles on the flat part of their rack, placed the called tile on the flat part of their rack or placed the called tile on the sloping part of their rack along with their other tiles.
A player calls a discarded tile while another player simultaneously racks the tile they picked; the player who called the discarded tile is entitled to it.
