Some reincarnate to become rectified, but they are not [entirely] careful and sin. Such a spark will become immersed again in the kelipot, with all the [minor] sparks that derive from it and depend upon it.
There are major sparks and minor sparks. The major sparks are composed of many minor sparks. They may reincarnate all together in one gilgul, or they may separate into many gilgulim.
What we are considering here is an intermediate case that includes both gilgul and ibur.
The Arizal is now going to describe the intermediate case of "ibur from birth," which is in some respects like ibur and in some respects like gilgul. There are parts of soul that reincarnate and come to a person when he is born, as is the case in gilgul. However, they have already been rectified, as in ibur, and like ibur they can reap reward from mitzvot that are done in this person's lifetime but do not suffer guilt from sins that may be committed. Thus, they are an intermediate category.
The reason for this phenomenon is that there are two types of blemishes resulting from sin. As a consequence of serious sin (which will be explained later on), the soul-sparks are severely blemished; they undergo gilgul. However, the result of less serious sin is a less serious blemish and does not necessitate gilgul.
A soul may reincarnate, achieve extensive rectification, but nevertheless remain somewhat blemished because of some "light" sins that were committed during the gilgul. These parts of the soul that are for the most part - but not completely - rectified will reincarnate again at the time of birth and remain with the person until the end of his lifetime, as in gilgul. However, in this case, they will reap reward for mitzvot but will not suffer guilt from sins that may be committed, as in ibur. It is these parts of the soul, those which are less seriously blemished, that constitute the intermediate category undergoing "ibur from birth."
The sparks of the nefesh do not separate from one another until the day of death
All the sparks of the Nefesh, even those that are already rectified, return in a complete gilgul at the time of birth with the individual spark that was damaged. They do not separate from one another until the day of death.
All the sparks of Nefesh -- both those that have been almost completely rectified, retaining only a light blemish due to a "light" sin, and those that have been only slightly rectified -- return at the time of birth with those soul-sparks that have been severely damaged and will require extensive tikun in this lifetime. Since they arrive at the time of birth, they are all similar to a "complete gilgul."
However, the rectified sparks reincarnate as an ibur.
In other words, the abovementioned almost-completely-rectified sparks reincarnate as an "ibur from birth." The reason it is called "ibur" is as follows:
They do not share in the sins of this body, only the merits. It is similar to the souls of righteous tzadikim who have died and come as an ibur during a person's lifetime and not at birth.
Even though the almost-completely-rectified sparks have to return with the damaged ones, they only benefit from the reincarnation.
Thus, a spark that has not been rectified whatsoever by the performance of those mitzvot to which it is related or [has not been rectified] as a result of transgressing those sins from which there is no revival must reincarnate into a second body, which will be named for that spark.
That spark's "name" is the essence of the gilgul that is happening, like the name of any given thing describes the essence of that thing. The gilgul reincarnates into a body for the sake of the tikun of those soul-sparks that have been severely damaged or not rectified at all. Therefore, that particular birth will be called by the name of those sparks that must undergo extensive tikun.
However, sparks that have achieved rectification through the performance of mitzvot, but have also been blemished through the transgression of "light" sins, only reincarnate as an ibur of the sort mentioned previously [i.e., ibur from birth], even though it is also a gilgul.
They are an ibur that must come back at birth, but they are not the main soul of the body in which they find themselves. It is because they are not the main soul of the body that they are an ibur. They are a gilgul because they come at the time of birth.
In contrast, there are sparks that have not been blemished through sins after having been rectified by mitzvot.
These sparks have been fully rectified and are not blemished whatsoever.
They do not return at all except as an ibur during lifetime [i.e., after birth], and then only if he merits it, as explained.
[Commentary by Shabtai Teicher.]