It is preferred that you place a called tile on the flat part of your rack and not in your rack among your tiles. This will prevent picking up the wrong tile for expous
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.
If a player inadvertently exposes the wrong tiles or exposes too many jokers, they can add to or take away from the current exposure as long as they have not discarded. After a player has discarded, exposures on their rack cannot be changed.
Tournament Rules are created to raise the competition level of the game and to ensure it is played fairly. Some groups enjoy competition so much, they play by tournament rules. Typically, tournament rules do not apply to social games.
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.