Normally, following the Abhidharmakośa, the celestial realm of immateriality (ārūpyadhātu: gzugs med khams) is said not to exist spatially and is said to be a pure mental/psychological state of existence. Note: gang la shi na de la ’o || “Wherever one dies, there is it.” But some Abhidharmic sources are said to maintain that there are indeed physical bodies and realms stacked above the akaṇiṣṭha realm. The designation “realm of immateriality” is thus to be understood as a “negative term being employed to [express] paltriness/meagreness” (dman pa la dgag sgra byas pa). Just as “five cents” would be regarded as “no money.” See Si-tu-paṇ-chen, bKa’ ’gyur dkar chag (p. 25). I don’t remember if the mChims chen or mChims chung makes this point.