The concept of hitlabshut ("enclothement"), on the other hand, implies a radical shift of focus in considering the nature of Creation. According to the perspective of hitlavshut, the chief dynamic of Creation is not evolutionary, but rather interactional. What this means is that higher strata of reality are constantly enclothing themselves within lower strata, like the soul within a body, thereby infusing every element of Creation with an inner force that transcends its own position within the universal hierarchy. Hitlabshut is very much a "biological" dynamic, accounting for the life-force which resides within Creation; hishtalshelut, onthe other hand, is a "physical" one, concerned with the condensed-energy of matter rather than the life-force of the soul.
Of all the innovative concepts which the Ari introduced into Kabbalistic thought, hitlavshut is the one which he identified as most significant. It provides the motif for his doctrine of the partzufim, wherein the sefirot are portrayed as complex interacting entities, cosmic "personae," which in "familial" constellation are constantly giving and receiving life-force from one another.
It is the theme of hitlavshut which informs the Ari's unique concern with the issue of gilgul neshamot, "reincarnating souls." Reincarnation is another manifestation of how one stratum of life-force can enclothe itself within another. The prime variety of gilgul, wherein human lifetimes themselves overlap, is described by the Ari as exhibiting the same pattern of hitlavshut as the overlapping of broader realms within Creation–the "legs" of the previous soul or lifetime enclothing themselves within the "head" of the present one.
We can now understand why the doctrine of gilgul does not appear anywhere within the system of the Ramak. Having not identified hitlabshut as part of his conceptual focus, the entire issue remains premature and in need of the Ari's future elaboration.
More information about Kabblah you can read in "What you need to know about Kabbalah"