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".

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.