Crystal Peak of Rong

...from The Light of Wisdom, Vol. 1, pg. 58:

These four syllables of OM, AH, HUNG, and HOH, briefly explained here, are called "vajra syllables of the Great Secret" because these cannot be discerned by the intellect of scholars and shravakas. These are also called "syllables symbolizing the tathagatas", like the syllables E and VAM. Among the two kinds of symbols--the ultimate itself manifest as symbol or sign and a symbol labeled by convention--these syllables are not artificially made into conventional words, but are the ultimate nature, which is spontaneously manifest as symbols. This is perceived and clarified by the wisdom of omniscience. As the life-force or embodied essence of all phenomena of samsara, nirvana, and the path, these are the basis from which all things manifest, the basis for exposition, and the vital point of recitation and meditation, as well as the ultimate destination.