The boundary problem is, in an important sense, the opposite of the combination problem. Recall that in the combination problem the problem is how the primitives of physicalisms — discrete atoms of experience (e.g. a conscious electron) — could combine their consciousnesses to create a macro-level consciousness like a human mind.
The boundary problem is the opposite. If one’s primitive is, essentially, a field (the whole of the universe or what have you), in which there is just one fundamental thing, you have the question of how one creates the “illusion” of an individual mind, eg an individual human mind, that is not the whole of creation.
Kastrup solves this problem with alters of dissociation. He quotes Rosenberg on this issue. “The challenge we must now tackle is the so-called “boundary problem for experiencing subjects” (Rosenberg 2004: 77-90): What structures in nature correspond to alters of TWE? We know that we humans do. Do animals too? What about plants? Rocks? Atoms? Subatomic particles? As Gregg Rosenberg put it, “we must find something in nature to ground [the boundaries of] an experiencing subject” (2004: 80) Kastrup p.71