What are the "seven stations of consciousness" supposed to mean?

DN 15: 7 planes of existence, what does it mean to have unified body and unified perception?

pali+english excerpt here: https://notesonthedhamma.blogspot.com/2020/02/dn-15-7-planes-of-existence-what-does.html

I had a similar question to OP, so I ask in here instead of starting a new thread. I'm also curious as to how it corresponds to 4 jhanas and their rebirth in brahma kayika realm on up.

It seems like being "unified in body" would probably mean they all look like clones of each other? From some other source, perhaps commentary, they say in brahma realm there's no gender, and no breathing (as humans, animals, devas have).

But what does it mean to be unified in perception? For brahma kayika, it could mean they're only interested in 4 jhanas and 4 brahmaiharas (rather than the diversity in deva realms where they could have differences in liking various divine pleasures of music, sex, food, etc). But then why is the next realm above Brahma kayika have diversity of perception?

Also Brahma kayika beings, some could be Buddhists, some Hindu, etc, so that seems like quite a diversity of perception right there.

After considering the pertinent answer of Mike Olds, I will provide a personal interpretation.

The Pali translated as "stations of consciousness" is "viññāṇaṭṭhiti", which is similar to the terms "viññāṇaṁ tiṭṭhamānaṁ" & "patiṭṭhā viññāṇassa" found in SN 22.53.

"Viññāṇaṭṭhiti" is explicitly described in SN 22.54 Bīja Sutta as follows:

Suppose these five kinds of plants propagated from seeds were intact, unspoiled, not weather-damaged, fertile, and well-kept. And there is soil and water. Then would these five kinds of plants propagated from seeds reach growth, increase, and maturity? Yes, sir.

The four grounds of consciousness (viññāṇaṭṭhiti) should be seen as like the earth element. Relishing and greed should be seen as like the water element. Consciousness with its fuel [nutriment] should be seen as like the five kinds of plants propagated from seeds. As long as consciousness remains (tiṭṭhamānaṁ), it would remain involved with form, supported by form, grounded on form (rūpappatiṭṭhaṁ). And with a sprinkle of relishing, it would grow, increase and mature.

If a mendicant has given up greed for the form element, the support is cut off, and there is no foundation (patiṭṭhā) for consciousness. If a mendicant has given up greed for the feeling element … perception element … choices element … consciousness element, the support is cut off, and there is no foundation (patiṭṭhā) for consciousness. Since that consciousness does not become established (appatiṭṭhitaṁ viññāṇaṁ) and does not grow, with no power to regenerate, it is freed.

Therefore, the seven grounds (ṭhiti) or establishments (patiṭṭhita) of consciousness (per SN 22.53, per SN 22.54 & per the post of Mike Olds) refer to the establishments/fixations of consciousness that are to be abandoned.

To the contrary, the 4th jhana is something not to be abandoned because, per the many suttas, the equanimity of the 4th jhana is directed towards the realisation of the Three Knowledges (of past abodes, the kamma of beings & the ending of the defilements); as also stated in MN 140, for example, where the equanimity of the 4th jhana was directed at liberation; or even in DN 16, when the 4th jhana was the last jhana entered before the Buddha emerged & passed away.

That is the end of my answer. The 4th jhana is not something to be explicitly given up because it represents the completion of the Noble Eigthfold Path that is to be directed towards Right Knowledge & Right Liberation.

As for the descriptions in the OP, I will retain the content of my previous answer to the question:

• The word 'kaya' does not mean 'physical body' ('rupa'). The word 'kaya' means 'collection' or 'group' of aggregates (as I have posted many times before. For example, when suttas use the phrase "at the ending of the body", this means the ending of a certain collection of aggregates rather than the ending of life).

• 1st jhana is more refined than the previous state of being. It is diverse in its collection (kaya) because it includes sankhara aggregate (vitakka & vicara) and feelings (vedana aggregate) as the settling place of consciousness. It is unified in perception only in comparison to the previous state of being (human, etc) because it has one-pointedness (ekkagattacitta). However, this unity is only illusory, i.e., only due to its comparison with the previous diverse state of being.

• 2nd jhana is unified in kaya because consciousness settles on only one aggregate, namely, feeling aggregate. However, it is diverse in perception because it knows two feelings, namely, piti (rapture) and sukha (happiness).

• 3rd jhana is unified in both kaya and perception because there is only one feeling as the settling place of consciousness, namely, sukha (happiness).

Piya Tan's analysis of DN 15 (on pages 160 through 163, i.e. page 16 through 19 of the PDF) explains that,

According to Brahma,jāla S (D 1) and Pāṭika S (D 24), this is not a “station for consciousness” because no consciousness is found in the beings there. All cognitive activities are suspended here. As soon consciousness arises in a being there, he vanishes from that world

It cites many references including e.g. DN 1 to begin with ...

There are gods named ‘non-percipient beings’.
Santi, bhikkhave, asaññasattā nāma devā.

When perception arises they pass away from that group of gods.
Saññuppādā ca pana te devā tamhā kāyā cavanti.

A STUDY OF VIÑÑĀṆA IN ABHIDHAMMA (by Ho Thi Lien) includes a description of what is meant by "unified in body" etc. (see pages 30 through 32 i.e. pages 42 through 44 of the PDF). It appears to be paraphrasing the DN commentary, for example:

‘Viññāṇaṭṭhiti’ means ‘abodes or supports of consciousness’, ‘the stages of sentient beings’. In the Dīghanikāya and its commentaries,⁸⁶ the stations of consciousness are classified into seven kinds as follows:

2) Nānattakāya-ekattasaññī = Beings who have different bodies but the same perception are Brahmās of the first jhāna plane who are born first in the Brahmā abode. Although their bodies and life spans are different, they have the same rebirth linking consciousness, which is the first jhāna. According to the Dīghanikāya Mahāvagga commentary, beings in the four apāyas are also included in this category because they possess different bodies but the same type of rebirth-linking consciousness, which is ahetuka akusalavipāka santīraṇa.

3) Ekattakāya-nānattasaññī = Beings who have similar bodies but different perceptions are those living in the second Brahmā abode. They have similar bodies but different perceptions. While in the first jhāna their minds are associated with both vitakka and vicāra, in the second jhāna, the minds are associated with vicāraonly but not vitakka; and in the third jhāna, both vitakka and vicāraare forsaken (according to the pañcaka method).

The footnote says,

⁸⁶ D. II, 58, DA. II, 99 -102, DṬī, 114 -116 (CSCD).

I presume the "eight liberations", which immediately follow in DN 15, also refer to these stages.

Piya Tan's analysis quotes the Paṭisambhidāmagga to say that the first few liberations are associated with meditations using internal and external kasinas (perhaps you'll disagree with that -- so far as I remember, perhaps I'm misquoting you however, you associate the jhanas with making "letting go" as the "object").