[4] Therein, which form (rūpa) is internal (ajjhatta)?
Whatever form, belonging to this or that being, is internal, personal, one's own, individual, clung-to (upādinna) — namely, the four primary elements (mahābhūta) and the form derived from the four primary elements (upādāya rūpaṃ) — this is called internal form.
Therein, which form is external (bahiddhā)?
Whatever form, belonging to this or that other being, to other persons, is internal, personal, one's own, individual, clung-to — namely, the four primary elements and the form derived from the four primary elements — this is called external form.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.