The process of rebirth, as described in several earlier posts, does not necessarily imply an interval between births, whereby the soul would sojourn in some other world or plane, but once we see a more fully evolved and developed soul with a complex mental and vital development, the importance and likelihood of such an interval becomes more probable.
Sri Aurobindo describes various scenarios that could occur as a preliminary to the deeper review he intends in this chapter. The first scenario we touched in a preceding post; namely, that the soul goes immediately from one physical incarnation to the next with no interval and no sojourn elsewhere. This explanation in fact might work for rudimentary developed souls that do not have the organisation or formed being that would be required to contact, move to, experience and return from worlds organised around the mental or vital powers. Evolution of the soul in this case would necessarily be slow and defined predominantly by the physical world and its needs and impacts. As the soul progresses however and develops a more powerful vital being and organisation, and a more complex and capable mental being, the issue of how these get assimilated, and built into a future incarnation becomes more salient. It is at this stage that we could see a new phase developing, wherein the soul goes to the mental and/or vital worlds with which it would have natural contact based on its prior development, and uses the interval to fully assimilate and prepare for the next phase in the soul’s development in a future incarnation. Some project also a third possibility, that a soul could be so evolved and organised as to be able to take an immediate next birth without the need for an interval, but through spiritual focus and choice, it could move seamlessly from one incarnation to another, pushing the evolutionary expression quickly forward.
Sri Aurobindo, The Life Divine, Book 2, Part 2, Chapter 22, Rebirth and Other Worlds; Karma, the Soul and Immortality