It is the client's turn to discard (or declare a kong, or declare tsumo). Default action: declare tsumo if possible; discard drawn tile (tsumogiri) otherwise.
A player has discarded a tile and the client can call the tile. Default action: call the tile for ron if possible; pass otherwise.
In the case of chow, the player may have to choose which other tiles to combine with.
A player has made an added kong and we can rob it. (This might be considered a special case of the "discard" state where ron is the only legal call and the turn does not advance to the next player afterward.) Default action: rob the kong.