The six-syllable mantra of Avalokiteshvara is the most widely reproduced string of Tibetan letters in existence. It is carved into mani stones, piled into mani walls along paths and passes, cut into rock faces, printed on cloth, written on paper and rolled by the thousand into prayer wheels. In many places it is the only Tibetan writing a traveller sees, and it is legible as writing to a great many people who cannot read anything else in the script.
Two things about it belong in an account of the letters. The first is that the mantra is public. It is displayed openly, it is reproduced commercially, and it is written both in Tibetan letters and, on formal surfaces, in Lantsa, the ornamental Indic script Tibet reserved for such purposes. Recording how it is written asserts nothing about anyone and withholds nothing from anyone.
The second is that the contemplative practice built on it is a different matter. Instruction in visualisation and recitation within the Vajrayana traditions is transmitted through a teacher and, for many practices, under formal initiation. The atlas describes the writing and the objects and does not reconstruct the practice, offer instruction, or present a syllable as something to be used. That boundary is the tradition, not a squeamishness about it, and the entries here observe it deliberately.