Kabe 「壁」 takes into account the number of tiles available (or unavailable) for tile groups, such as sequences and triplets. Both tile efficiency and defense are in consideration. In the case of tile efficiency, certain tiles may not be available; or they're available in fewer number. As for defense, visible tiles may help indicate and deduce which tiles are safer to discard than others. Of course, this technique may or may not effectively deduce 100% safe tiles.
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.
Calls for kan have an astute effect on kabe, as this tile call requires all four of a single tile type. So immediately, this renders particular tiles as unavailable to the other players.
By taking note on the unavailablity of certain tiles, then one can deduce the impossibility or less likelihood of certain tile groups to form. For example, when two of a type type are visible, then another player cannot form triplets. This is particularly true for honor tiles. The same logic may be applied to sequences. The unavailability of certain numbered tiles increases the difficulty in forming certain numbered sequences.
As a consequence, the unavailability of certain tiles may hinder one's hand from either tenpai, completion, or even forming certain yaku. This is particularly the case for yakuman. In the case of kokushi musou, if all four of a tile type required for that yakuman, then the possibility for any player completing the hand is completely nullified.