Spiritual Realms
There are three Spiritual Realms that the soul can be in. I will abbreviate these as:
SR1: The Originator (Creator, God) in its first form, ultimate in its power, knowledge and perfection and separated from its other forms that are within the Universe, to maintain its perfection (this is the first heaven).
SR2: The spiritual realm within the Universe, but separate from the planetary spiritual realms, the soul’s spiritual domain and all that is material/physical (this is the second heaven).
SR3: The earths and other planetary spiritual realms. These permeate the planets and surround it (these are the third heaven).
NB: The soul of course, is a part of SR1 although separated from it. Heaven is within you.
Spiritual Realm 1 (SR1)
The barrier between SR2 and SR1 can only be crossed when the soul is perfected.
As you can see, Spiritual Realm 1 is the Originator in its highest form.
Spiritual Realm 2 (SR2)
Spiritual Realm 2 is within the Universe, but outside of earth’s spiritual realm.
SR2 is also the first prison.
A soul cannot cross into SR2 until its connection with the physical body is broken; a break is permanent. When the physical body dies the soul/body connection is lost, it is then that the soul enters SR2 by its own will. In some circumstances, a soul may choose not to cross into SR2 and instead, stay in SR3. A soul cannot cross back from SR2 into SR3, until it has chosen an available new physical body.
Spiritual Realm 3 (SR3)
Spiritual Realm 3 surrounds each planet that is inhabited by the souls that are the children of the Originator.
SR3 is also the second prison.
The physical body is the third prison, though the soul may leave when it desires. When a soul leaves its physical body, and yet it is still connected to it, it is confined to the spiritual realm that surrounds the planet, SR3.

When a species travels from one planet to another, SR3 encompasses the traveller, so maintaining the barrier between SR3 and SR2.