In the Cassiopaea and Ra materials, density denotes a qualitatively distinct level of being. Each density has its own structure of life forms, perception and typical lessons for the consciousnesses residing in it.
Densities are defined roughly as follows:
1st density - Inanimate matter. If we can speak of learning here, the lesson is maybe for matter to learn to combine in ways forming primitive biological organisms.
2nd density - vegetal and animal kingdom - Lessons have to do with biological life, survival, adaptation, competition, group organization as seen with animals and the like. The soul structure is generally a species soul pool, but as species become more advanced, individual members of the species may differentiate themselves by more varied individual learning.
3rd density - human - Lessons of 2nd density continue in more complex forms, as in battles for social domination, more survival etc. Individually acquired personality and individual learning play a greater role than in 2nd density. Humans are divided into preadamics, who share a species-like soul pool and Adamics, who have an individuated soul.
The specific lesson of third density is however making a choice of orientation of service, towards service to others in its greatest possible manifestation or service to self in its greatest possible manifestation. Making such a conscious choice requires having an individuated soul and presumably a large number of lifetimes in third density for the soul to acquire its polarity.
4th density - This is described as a partly physical state where graduates of third density may deepen and perfect their chosen polarity. STS and STO groups are distinct in fourth density and do not automatically come in contact, unless in the context of interacting with 3rd density.
Most of the UFO phenomenon involves 4th density service to self beings. Pure service to self may not occur past the 4th level of density, presumably because this is the last at least partly material density.
4th density beings enjoy more conscious control over physicality and generally form groups telepathically sharing a common pool of experience while retaining a certain individuality.
5th density - Souls of 1st thru 4th density find themselves in 5th density between incarnations. This is a contemplation zone where these souls may observe their past/future lives from a purely ethereal state of being. However, for progress to be realized, the souls must incarnate in the density which best corresponds to their level of progress.
6th density - This corresponds to the level of 'names of God' or 'unified thought forms.' Service to others entities who no longer need to reincarnate occupy this level. This corresponds to angels or Dhyanic beings in other terminologies.
7th density - This is the level where all is one and one is all, in a practical, real and meaningful sense. There is no longer any difference between thought and reality. This corresponds to a notion of all encompassing god or universe or the Sun Absolute of the 4th Way.
The concept is complex and words of 3rd density are not fully appropriate for most aspects of the matter. The Ra material offers a somewhat systematic overview of the question. The Cassiopaea material builds further on that basis. There are slight differences of emphasis and definition between the two but from our standpoint the essential message is that a passage from 3rd to 4th density is in some cases possible and requires first mastering the lessons of 3rd density, specifically survival, karma and formation of a truly individuated being capable of consistent work in one's chosen polarity. The mastery of self spoken of by the 4th Way is a subtask of these.
The different cosmoses of the 4th Way cosmology can be seen to correspond with densities. Direct comparison is difficult because most of this material lies outside human experience.