What does Nonduality correspond to in Buddhism? There is not [was not] a tag for nonduality. Would someone please make a correspondence between nonduality and Buddhism as to "stage" or "attainment", qualifications, or whatever is applicable?

EDIT: I was thinking of Nonduality as a stage, but it is apparently seen more as a position or way of describing things? Mariana Caplan, in the book "Eyes Wide Open - Cultivating Discernement on the Spiritual Path" has this paragraph at the top of page 163 (paperback): 


  Ngakpa Chogyam, a Tibetan Buddhist teacher from Wales, offers a
  perspective on nonduality that includes all of life as a direct
  expression of the nondual core of truth. He explains that nonduality,
  or emptiness, has two facets: one is empty, or nondual, and the other
  is form, or duality. Therefore, duality is not illusory but is one
  aspect of nonduality. Like the two sides of a coin, the formless reality has two dimensions -- one is form, the other is formless. When
  we perceive duality as separate from nonduality (or nonduality as
  separate from duality), we do not engage the world of manifestation
  from the perspective of oneness, and thereby we fall into an erroneous
  relationship with it. From this perspective it is not "life" or
  duality that is maya, or illusion: rather it is our relationship to
  the world that is illusory.


This accords with the Heart Sutra. So, I was actually asking about the Experience of this, rather than whether it is true or not. "Both is, and is not. Neither is, nor is not." (Buddha)

Second Addition: I find a correspondence between the formal / post-formal operations distinction and the observation that some people get stuck when thinking of abstractions like nonduality, and others do not. Some people are more literal and fundamental, and others are more mystical. I think this is the key to understanding differences, and post-formal thought is an ability that develops through use. Here is a link from a teacher's experience.
There's an article on that subject Dhamma and Non-duality by
Bhikkhu Bodhi.

The following is basically all direct quotes from that article, except very summarized (I'm extracting sentences and sentence fragments).



Non-dual system

For the Vedanta, non-duality (advaita) means the absence of an ultimate distinction between the Atman, the innermost self, and Brahman, the divine reality, the underlying ground of the world.

The Mahayana schools, despite their great differences, concur in upholding a thesis that, from the Theravada point of view, borders on the outrageous. This is the claim that there is no ultimate difference between samsara and Nirvana, defilement and purity, ignorance and enlightenment. For the Mahayana, the enlightenment which the Buddhist path is designed to awaken consists precisely in the realization of this non-dualistic perspective. 

Not a non-dual system

As for the Theravada tradition:


Virtue


Non-duality: the adept isn't bound by rules because "The sage has transcended all conventional distinctions of good and evil"
Theravada: "the liberated one lives restrained by the rules of the Vinaya, seeing danger in the slightest faults"

Meditation:


Non-duality: defilements are mere appearances devoid of intrinsic reality
Theravada: hindrances are "causes of blindness, causes of ignorance, destructive to wisdom, not conducive to Nibbana"

Wisdom:


Non-duality: concrete phenomena, in their distinctions and their plurality, are mere appearance, while true reality is the One: either a substantial Absolute (the Atman, Brahman, the Godhead, etc.), or a metaphysical zero (Sunyata, the Void Nature of Mind, etc.). For such systems, liberation comes with the arrival at the fundamental unity in which opposites merge and distinctions evaporate like dew
Theravada: wisdom not in the direction of an all-embracing identification with the All, but toward disengagement and detachment, release from the All





So to answer your question, if I understand the article, it's saying that "non-duality" in Buddhism includes statements like "nirvana and samsara are the same" and "everything is equally empty" ... but non-duality is not a feature of Theravada Buddhism (it's a feature of Mahayana Buddhism, and of Tantrayana).