The discarded 6 of Sticks can be used by Player to form a Chow. This could also complete mahjong for Player. However, if Player were trying to build a higher scoring hand, they might not want to go mahjong yet.

The left green button will accept the tile to meld a Chow. Play would continue with the Player discarding.

The right green button will accept the tile and complete the mahjong. You would see Win by Discard next.

The red button will reject the tile and play will continue immediately.