When astral traveling there are different forms the mind, body ad conscience can take.

These forms are called the ''subtle bodies''.
Subtle bodies differ in density; how close together the particles in their makeup are.

The subtle bodies also range in ability and mobility.
There are 5 subtle bodies, let us take a closer look at them.

The least densest, the Casual Body or ''abstract mind'' is exactly how it sounds. ''Abstract'' meaning far from reality or not concrete. This body can not venture any further than the mind/brain itself, much less actually leave the body. You are in this form when simply dreaming.

The second to least dense would be the Mental Body or ''concrete mind''. ''Concrete'' meaning very close to reality. This subtle body also can not move further than the brain (unless specifically trained to do so, which is in a different lesson). You are in this form when you are what you will know as lucid dreaming.
~ Where mental projection takes place.

The middle or 3 densest subtle body is know as the Astral Body. This is the form most often used during astral travel. It may move to any plane higher/above than this one (i.e I can only ascend from this plane it can not descend from this plane), this also means it may not travel on this very plane.
~ Where astral projection takes place.

Then we have the second densest subtle body, the Etheric Body. The Etheric Body is very dense, so much in that some people may be able to see it. This does in fact mean it may travel on this plane, as well as planes above and below it.
~ Where etheric projection takes place.

Last subtle body really isn't subtle at all, the Physical Body. All are familiar with this one and that it can not move beyond this plane. The body of which has skin, bones, organs etc...

--

Now for a bit of clarification on the subtle bodies.

The Astral Body may not descend and or stay on this plane. But that doesn't mean it may not be able to view or travel as though it were. There are planes that exist primarily right on this one, that they exist intertwined or rather on the same level. For instance mixing colors of paint, there are still atoms of both colors that are not apart of the other color but the 2 colors still share the same surface. What is on this plane can be viewed from this plane, what is not on this plane can not. A person in the form of the astral body may only be able to view this plane and those on this plane not be able to view them. The 2 planes be on the same surface but this plane is still considered below it because it must be traveled through to get to any of the lower planes.

High planes- planes resting on or above the physical plane (this plane)
Low planes- planes resting below the physical plane