This discourse is the same as the first section of DN 20.

The Pure Abodes (suddhāvāsā) are realms into which only nonreturners can be reborn. They are reckoned as five: Aviha, Atappa, Sudassa, Sudassı̄, and Akaniṭṭha.

Read khīla (“post”) rather than khila (“barrenness”).

A village (Bu Pj 2:3.8) or royal compound (Bu Pc 83:1.3.56.1) was marked with a “boundary-post” (indakhīla). It symbolized a fixed and immovable point, either in a good sense (SN 56.39:4.1) or, as here, an obstacle. The Sanskrit indrakīla is a two-foot long iron bolt securing a fort, presumably fixed in the ground (Arthaśāstra 2.3.26), or else a mountain in the Himalayas. The warrior Arjuna, journeying beyond the Gandhamādana, is stopped at the Indrakīla mountain by Sakra/Indra disguised as a brahmin ascetic, at whose urging Arjuna undertakes asceticism in order to gain the power he would need (Mahābhārata 3.3.37). The symbolic sense of “stopping” relates to the Pali sense of immovability, and suggests that the underlying metaphor is the irresistible power represented by Indra.

Nāga applies to powerful beings: the cobra, the elephant, the dragon, and as here, the spiritual “giants” of the Saṅgha.