Substance ought to be easy. 'Thus I assert thee' as I stumble over a stone, as it were. That's fine but then we are obliged to distinguish between 'stone' as stuff, generic stone and this particular stone called by Aristotle the primary substance and the former the secondary. By the way none of the interpretations of Aristotle's views are uncontroverted so shop around. However the primary/secondary division seems to me a sensible view of the basic furniture of the world or the floor we start from. The Advaitins who employ different criteria of what really is regard 'stuff' as more fundamental than individual items. In their airy way they assert that 'clay alone is real, things made from clay are mere names'. The changeless element, the material cause in the Aristotelian schema, is the clay and the various forms that are made out of clay are simply conventional names. We can see this view in some Western ontologists who would regard a table as 'timber arranged table-wise'. Is this a monist view which would regard our primal encounter as being with an undifferentiated continuum? Given evolution this seems unpersuasive as animals seem to be 'designed' to fit a habitat and prey. For the fox is a rabbit 'food arranged rabbit-wise'?
The pramana of upamana seems to contradict the primacy of 'stuff' (cf various posts on the topic of upamana by label). Recognising genus and species is a basic function of this innate means of knowledge ie. a means of knowledge not reducible to any other. Once we have decided on the criteria of division we can place everything. Encountering anything precipitates automatic sorting on however crude a basis. True but for the Advaitins their concept of 'the real' as changeless supervenes so the opposition may be more a contrast than a contradiction.
The contrast that exists between everyday awareness and knowledge and the reality that underpins that knowledge is brought out in the concept of the primal reality of the substratum. At the risk of confusion it is arguable that the clay/vessels dichotomy with clay alone as real is merely an analogy for the reality of the substratum and the unreality of the upadhis/forms of limitation which the substratum manifests.
Or the characteristic of Brahman is Its being the material cause of the entire universe. By 'material cause' is meant the substratum of the superimposition of the universe, or the substratum of the cosmic illusion (maya) that transforms itself in the shape of the universe. It is in view of such material causality that Brahman and the universe have been described as identical in Sruti texts like the following: “This all is the Self” (Br.II.iv.6) “It became the gross and the subtle (Tai:II.6) and “Let Me multiply, let Me be effectively born (Ibid II.6, Ch.Up. VI, ii, 3) Conventional statements like, “The jar exists,” “The jar is manifest,” and “The jar is desirable,” are also on account of the superimposition of its identity with Brahman, the Existence-Knowledge-Bliss Absolute.(from Vedanta Paribhasa tranlated by Swami Madhavananda.)