The rules are made by people, so the rules vary from place to place. For example, when playing mahjong in some places in Hunan, there will be only one card left as the bottom of the sea, and in some places, there will be the last two cards left. So it depends on what the local general rules are.
The hidden bar is a mahjong term, which means that a player has four identical cards in his hand when playing cards, such as four ten thousand cards and four nine barrels.