The Japanese esoteric schools assign to each of a large set of buddhas, bodhisattvas and other figures a Siddham syllable, called in Japanese the shuji, and the syllable can stand for the figure anywhere the figure would otherwise be named or shown. It appears on images and their bases, on memorial tablets, on ritual implements, on amulets and on mandalas, and there are mandalas composed entirely of syllables rather than of figures.
The assignments are recorded in reference charts that form a standard part of the shittan literature, and they are stable enough that the same syllable is generally the same figure across the tradition. Many of them are not bare letters. A seed syllable is frequently a letter with a vowel mark, a nasal mark or the visarga added, so what stands for the figure is a written syllable rather than a character from the table.
Two things need saying alongside that. Recording which syllable belongs to which figure is a description of what is written, and it is publicly documented in the objects themselves. The contemplative and ritual practices these syllables belong to inside Shingon and Tendai are a different matter: they are transmitted from teacher to student under initiation, and the atlas neither sets them out nor treats a syllable as something a reader is being handed to use.