what is the "density" exactly means?

[0lin]

The alleviation mechanism was designed by the placement of this consciousness in second-dimensional physical chemical complexes which are
not able to be dextrous or manipulative to the extent which is appropriate
to the working of the third-density distortions of the mind complex.

this doesnt seem to imply dimensionality like that.

it basically says, the advanced spirits were put into less capable bodies.

I didn't say it imply any dimensionality. I said it imply the body of monkey is a body of 2nd density.

What is this means that human is the entity of third density or animal is the entity of second density? Is this means that the mind or the body is in the specific density?

For the 2nd density animal, the mind and body are in 2nd density.
For the 3rd density human, the mind and body are in 3rd density.

The bodies of both beings are in space/time, while the minds are in time/space.

Each higher density subsumes the lower's qualities.

Does a fourth density entity have a body like us?

I know that they can and do have bodies like us. This body is apparently difficult, but not impossible, to manifest in our 3rd density. For 5th density, it is apparently simple.

Or they don't need one because they live in the time/space dimension.

They are still centered in space/time, within their dimension. However, 4th-density time/space is much more transparent to them than our 3rd-density time/space is to us.

I think this is the right answer by adding that 4th density body could be seen in our space/time. I think it is also a 3 dimensional body and there's no 2 or 4 dimensional bodies. Ra use the term "density" just like us. A 3rd density body could have a 3 dimensional consciousness, while 2nd density body could only allow a 2 dimensional consciousness and the body of 4th density allow a 4 dimensional consciousness to live in.
But i still don't understand the following materials:
Book III,session 63
"As the green-ray cycle or the density of love and understanding begins to take shape the yellow-ray plane or Earth which you now enjoy in your dance will cease to be inhabited for some period of your space/time as the space/time necessary for fourth-density entities to learn their ability to shield their density from that of third is learned.
Meanwhile there is another sphere, congruent to a great extent with yellow ray, forming. This fourth-density sphere coexists with first, second, and third. It is of a denser nature due to the rotational core atomic aspects of its material. We have discussed this subject with you. The fourth-density entities which incarnate at this space/time are fourth-density in the view of experience but are incarnating in less dense vehicles due to desire to experience and aid in the birth of fourth-density upon this plane."

It is the most confusing thing that the 4th density entities will learn to shield their density from the 3rd and the last word "plane" imply that the 3rd and 4th density may not in the same dimension? Could anyone interpret this?