Three syllables stand in Tibetan Buddhism for enlightened body, speech and mind. They are written on images, printed in texts, set on the reverse of paintings and inscribed on ritual objects, and the second and third of them are built on letters of the ordinary alphabet: the great a carries the second, and ha carries the third.
The writing is public. It is displayed openly, reproduced commercially, and shown in printed reference works and museum catalogues, and describing how the syllables are written asserts nothing about anybody and discloses nothing that is withheld.
The practice is not the writing. Within the Vajrayana traditions, the contemplative use of these syllables is transmitted by a teacher and, for a great many practices, under formal initiation, with the instruction given to those who have received it and not otherwise. That is a boundary the traditions themselves draw, and it is not a matter of secrecy for its own sake: the instruction is held to depend on the transmission.
The atlas observes it. These entries describe letters, shapes, and the objects the syllables appear on, and they record what is documented about their iconographic use. They do not reconstruct a practice, do not give instruction, and do not present a syllable as something a reader should use. Where a practice is closed, the atlas says so and stops, which is the only honest position available to a reference work.