The division between openly taught and empowerment restricted mantras described elsewhere in this entry is not an ad hoc distinction; it follows a formal fourfold classification of tantra found across Tibetan Buddhist scholastic tradition, Kriya, Charya, Yoga and Anuttarayoga, the last usually translated as highest yoga tantra, ranked according to the type of practitioner they suit and the intensity of the methods they employ. Mantras and practices belonging to the lower classes, Kriya and Charya tantra, have historically been taught more openly, with Tibetan tradition generally holding that their methods carry less risk if approached without full preparation, which is part of why formulas like Om Mani Padme Hum and the Tara mantras, both associated with practices in this more accessible range, circulate freely. Anuttarayoga tantra, the highest class, is where the material genuinely restricted to empowerment recipients concentrates, involving practices Tibetan tradition holds require a qualified teacher's direct transmission and ongoing guidance precisely because of their greater intensity and, traditionally, greater risk of misunderstanding or misuse without it.
This fourfold scheme is itself a scholastic systematization developed within Tibetan monastic tradition to organize a very large and historically accumulated body of tantric material, and different lineages within Tibetan Buddhism draw the practical line between open and restricted teaching at slightly different points even while broadly agreeing on the underlying four class structure, so which specific practices count as anuttarayoga rather than yoga tantra is itself a matter some schools treat differently.