Superimposition / Adhyasa as manifest in ordinary Awareness. Upadesa Sahasri 8

54. The teacher said, Listen. It is true that the Self and the body are well-known, but they are not well-known to all people to be objects of different knowledge, like a human being and a trunk of tree. (Question): How are they known then? (Reply): (They are always known) to be the objects of an undifferentiated knowledge. For, no one knows them to be the objects of different knowledge saying, "This is the body" and "This is the Self". It is for this reason that people are deluded about the nature of the Self and of the non-Self and say, "The Self is of this nature" and "It is not of this nature". It was this peculiarity with reference to which I said that there was no such rule (viz., only well-known things could be superimposed on each other).

(from Chap.2 Upadesa Sahasri)

What Shankara is doing here is moving the concept of Superimposition to a new locus viz. to human self-awareness. The Self as encountered empirically is not known as separable from the physical entity through which it manifests. Consciousness is always manifested under some form or other. We can conceptually grasp the reality of the Self. However this apprehension is always in some state or other of manifest awareness. This is the basis of the pratibodha videtam (known with reference to each state of awareness) as delineated in Kena Up. II.4. cf. my previous remarks:
pratibodha videtam