I want to explain the detailed meaning of this.
By default people think of ‘everything’ as a single category of things.
Eternalist thinks the category of everything is a classification of all things which are mind-made constructs, they think that mind makes world and can be without the world.
Annihilationist thinks everything is essentially a classification of the universe, the world, nature, and mind that is made therein, and that the world can be without it too.
For both of this
Everything = Reality (either the mind or the universe)
Ariya say that mind & world are dependently originated and not otherwise.
The discernment of a mind requires the discernment of a world and the discernment of a world requires the discernment of a mind.
This duality is on one side.
Asankhata is on the other side.
Therefore for the ariyan
Everything breaks up into two categories of discernable Reality
Category1 - Reality which is of mind & world
Category 2 - Reality where neither this world nor another
Therefore to them there is also that which is described as reality & everything which the puthujjana misunderstands but there is also an analogical category of reality delineated & divorced from the former.
So one gets two categories of principial reality, one which changes and one which doesn’t.
And this is not eternalism because the eternalist thinks there is only one pricipial reality which having been changing becomes stable & doesn’t change anymore.
And this is not annhilationism because the annihilationist thinks there is only one principial reality wherein the absence of something that was is stable & doesn’t change.