Kabe

Kabe 「壁」 takes into account the number of tiles available (or unavailable) for tile groups, such as sequences and triplets. All tiles shown in the discards, dora indicator(s), and even one's own hand. These tiles are noted as "visible tiles". Players take into account, that every tile type has three copies plus its own. In other words, for every tile, there are four of them. With that in mind, for each specific tile type, players can keep track of how many are left available, as the discard piles grow, along with the tiles in the hand.

Both tile efficiency and defense are in consideration. In the case of tile efficiency, certain tiles may not be available; or they're availalbe in fewer number. As for defense, visible tiles may help indicate and deduce which tiles are safer to discard than others.