Japanese esoteric Buddhism works with two great mandalas, taken from two texts, and they are used together as a pair. The Womb Realm mandala descends from the Mahavairocana Sutra and the Diamond Realm mandala from the Vajrasekhara, and the two were transmitted as a single system by the masters at the Tang capital from whom Kukai received it. Shingon temples hang them facing each other.
Mahavairocana stands at the centre of both, and each arrangement gives him a seed syllable of its own: the letter a in the Womb Realm, and the letter va with the nasal mark in the Diamond Realm. The pairing of the two syllables mirrors the pairing of the two mandalas, and it is one of the clearest cases of the Siddham letters carrying structure rather than merely labelling something.
Mandalas composed of syllables rather than of figures exist for both, with a seed syllable in each position where a figure would otherwise stand. Such a mandala is fully legible to somebody trained in the tradition and completely opaque to anyone else, which is a fair description of the whole apparatus.
What the atlas records here is the arrangement and its documentation. The use of these mandalas is initiatory, transmitted within a teacher relationship, and describing what is depicted is not the same as describing what is done.