What are the "seven stations of consciousness" supposed to mean? DN 15 includes the utterance of the "seven stations of consciousness", where the later six equate with the six of the eight jhanas, namely: 


  1. There are sentient beings that are diverse in body (kāyā) and diverse in perception (saññino), such as human beings, some gods (devā) and
  some beings in the underworld.
  
  2. There are sentient beings that are diverse in body and unified in perception, such as the gods reborn (ābhinibbattā) in Brahmā’s Group (brahmakāyikā)
  through the first absorption
  
  3. There are sentient beings that are unified in body and diverse in perception, such as the gods of streaming radiance (ābhassarā).
  
  4. There are sentient beings that are unified in body and unified in perception, such as the gods replete with glory (subhakiṇhā). 
  
  5. There are sentient beings that have gone totally beyond perceptions of form (rūpa). With the ending of perceptions of impingement, not
  focusing on perceptions of diversity, aware that ‘space is infinite’,
  they have been reborn (upagā) in the dimension of infinite space.
  
  6. There are sentient beings that have gone totally beyond the dimension of infinite space. Aware that ‘consciousness is infinite’, they have
  been reborn (upagā) in the dimension of infinite consciousness.
  
  7. There are sentient beings that have gone totally beyond the dimension of infinite consciousness. Aware that ‘there is nothing at all’, they
  have been reborn (upagā) in the dimension of nothingness.


The deva (godly) abodes attained with each rupa jhana are listed in AN 4.123, namely, brahmakāyikā, ābhassarā, subhakiṇhā &amp; vehapphalānaṃ. 

Now, the above utterance in DN 15 omits the 4th jhana or gods of abundant fruit (vehapphalānaṃ). 

My question is what are the "seven stations of consciousness" supposed to mean (for example, why do the permutations of 'diversity' and 'unity' exist in the various jhana &amp; states of being) and why, in particular, does this utterance omit the 4th jhana?
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").