As for the divisions of the three inner tantras:
There are three divisions within anuttara tantra,
These are the father and mother and the non-dual tantras.
Respectively each of these emphasizes the stages
Of developing and perfecting and the union of these.
These three yanas are known as maha, anu, and ati.
The deities may unite, and in holy substances
There is no distinction of what is clean or defiled.
Rather it is taught that all is of one taste,
Regarded as the play of a single mandala.
The Guhyasamaja and the Mañjushri Yamantaka and such yoga tantras are mahayoga tantras, the aspect of the skillful means of the developing stage. The completion stage of mahayoga chiefly teaches prana.
Yangdak, Vajrakilaya, Chakrasamvara, Hevajra, and so forth are prajña mother tantras. These are called anu yoga. Chiefly the completion stage of prajña is taught. The completion stage also chiefly works with the elements of bodhicitta and bindu, which are maintained to be the completely non- conceptual wisdom of bliss and emptiness.
The non-dual tantras, The Great Net of Illusion (sgyu ‘phrul dra ba chen po) and so forth, are called ati yoga. These chiefly teach the essence of prajña and upaya without adding and taking away, the union of the developing and fulfilling stages. In the completion stage, from prana, nadi, and bindu are created bliss, luminosity, and non-thought, and the inconceivable luminosity of the great wisdom.
Each of these three, as a symbol of the union of upaya and prajña, visualizes the embrace of the mother and father deities. The meaning of this is being without accepting and rejecting, denying and asserting, clean and defiled, and such accepting and rejecting of good and bad. Therefore there is enjoyment of samaya substances such as meat and liquor and so forth, and it is maintained that all dharmas are one in the enlightened mandala of primordial buddhahood. The king of the tantras that ascertain the suchness of the secret essence, the Great Net of Illusion says:
Since it is the characteristic of all dharmas that they have a single essence with primordial buddhahood, they are inseparable from that.