The term appears in the 'Five Groups of Dharma', according to Pabongkhapa Déchen Nyingpo [REF].
In the Nyingma school of Tibetan Buddhism, Anuttarayoga tantra is sometimes used as a synonym for the Mahāyoga tantra of their nine-yāna formulation, wherein six levels are articulated in two triads, the 'Outer' and 'Inner' tantras.