1. DUALITY - is the idea of "real opposites". That there really is good and evil, black and white, up and down, male and female etc. The differences of objects are believed to be "real" and a first-hand reality. That when you see these things you are looking at reality. They I really am my body, that you really are your body. That there is no unity at all, no connection, no joining, everything really is separate. In the Course, this is equated with "false perception." And in fact Jesus says "the world is false perception", meaning that the physical objects/forms themselves ARE the idea of separation, without which they would not exist at all.
2. NON-DUALITY - the original form of the non-duality idea is that all of the objects present in duality "are there" still, but there is an underlying oneness/unity which joins them all together. It's as if to say, God is a limitless being who is "expressing as" various objects. As if a clock and a chair are "really" one thing, as if joined by a hidden energy field of field of mind. That "all things are one".
In this view, the world, bodies, Earth etc are actually "kept" and still show up and move around seemingly independently etc, but are thought to be all part of one overall single Mind's thinking, or its self expression. As if God is animating the world. As if God Himself is the world or is "in it", like in the background, pulling the strings. In this view, the world is retained but is reinterpreted as "one thing", wherein its various parts and shapes seem only to be separate things when the mind is unaware of the unity it springs from.
That you might in some way "experience it all at once" if an experience of oneness, a kind of transcendent unity in which "you are the airplane" "you are the car" "you are the house". Here, God seems to be in everything you see. This idea is actually not really in ACIM at all. Some people believe it is. But ACIM actually teaches that reality is BEYOND the forms, not AS them, regarding the forms themselves to be illusions regardless of how they are experienced - that even experience as "all one thing" is an illusion. This is often confused with "true perception" which is similar but not the same - in true perception see reality beyond forms, not as "their true nature".
3. PURE NON-DUALITY - as touted by Ken Wapnick and several others, is a progressive form of non-duality, takes the aspect of non-duality that seems to still be "a world" with things happening, and says that all of these worldly things are literally 100% not there. Not even that they are reinterpreted or experienced as "one thing". Not even that you are "experiencing them all at once" - it separates the "oneness" from the "things" entirely and labels the things as absolutely nothing - non-experiencable. In pure non-duality, the idea is that there is "only God", and nothing else. Only the oneness. Seemingly the oneness alluded to by non-duality but dispensing entirely with the world of forms and all objects and bodies and happenings. This does tie in with the Course teaching that "there is no world". Not simply that "the world is one thing" but that it is NOTHING.
A pure state of "only God". Just an expansive eternity is sameness, no change, no movement, no creating, no increase, no individuals, no objects, no forms whatsoever, no differences, no appearances of any kind, just rock solid God on his own. In this view, logically, it should also mean that there is no Son of God and no Holy Spirit. Some people sometimes actually "go there". That we are God. That only mind exists. That even the Son doesn't really exist as anything other than God's radiance, still solely God.
Some have used this to justify that the Holy Spirit does not exist, that Jesus does not exist, etc. It's often associated with the idea of "Oneness". That there is only "one thing". Literally a count of "one". One box, one entity, one being, just God all by Himself. This view ignores creation, sonship, free will, souls etc. This view is really not in ACIM **AT ALL**, but many think it is. It's actually the result of the ego's reasoning, believing that if you remove all separations and get down "one" thing, it must be the final answer about what exists. This essentially boils down to the idea that God is alone and is dead because he is not extending, creating, sharing or loving. He just "is" ("God is"), WITHOUT anything having been created (no sonship, no souls, no creations). It logically becomes the idea that God is dead, because love has to be shared and has to create in order to exist.
4. TRANS-DUALITY - I don't know if there is really a word for this. It takes the idea of pure non-duality, in which there is NO WORLD (only Heaven exists and Earth is not it) but then acknowledges that "God did not will to be alone". That God is a creator. That God loves to ADD to His "solo" reality. That God has in fact created, not just one thing, but a great many things. That the Holy Trinity is real, there really are three parts, and the Sonship (as one of them) actually contains many souls that God created to freely create WITH him. This is much more closely aligned with Christianity.
All of these creations, together "the one son" in ACIM, aka "sonship", are extensions of His mind and being and have been fully created as essentially autonomous entities, with their own minds. However, this is not the same as in duality. In duality these individuals would be cut off from each other, ie separate. In "trans-duality", these entities share everything with each other. AND YET, do not lose their individuality. They share such extensive oneness with each other, you would think there is nothing defining their "self", and yet there is. God has fully created souls. It defies ego thinking. It is miraculous.
In this model, which is really the model of God being a creator who has created a Sonship (and the Holy Spirit) as eternal beings, "The Kingdom of God contains all his children and their creations", these beings are co-creators with God, and have their own creations, which are shared and celebrated by the whole. This is an extraordinary state where everyone shares absolutely everything with everyone else, including God. Total access. Total openness. No selfishness, no exclusion, no ego, no separation ideas, no thoughts of imprisonment or unwillingness or denial.
A family of God, sharing in love endlessly, celebrating and being thankful for having been created, joining with all brothers in a union which seems to transcend their self yet allows them to still have one. "The ego is the idea of selfishness, the soul is the idea of self fullness." It is the miraculousness of the soul that allows it to share everything without ceasing to exist. God created many children who are permanent brothers to each other.
God is a holographic thinker and creator - every part is a whole in its own right. He has created beings, "in number infinite". THIS IS THE TRUE MODEL OF A COURSE IN MIRACLES. It acknowledges God as Father and you as A SON, one of many. This is not non-duality, it is not pure non-duality, it is not duality. It is a state of "many and one" at the same time. It is God's family.
God's system is a system of inclusiveness, which is why all beings He created share everything. It's a system of AND/BOTH rather than the ego's system of EITHER/OR. All children of God are equal, per the Golden Rule, and are brothers. "One brother is all brothers. One mind is all minds." This does not in any way mean there is only one mind or only one brother. Nor does it mean they are not joined as if one. It is a fusion of oneness and many-ness, without contradiction, and with no ego involved whatsoever. It is not a separation idea, is not duality, or non-duality. It is the Creation of an omni-present being who is fully present in all things He creates without limit, making each creation a whole in itself. It is a system of "many wholes", which defies all ego thinking.
This is what A Course in Miracles is teaching. This is reality.