[The soul of] Adam divides into 248 "limbs" just does his body. In addition, there are other aspects, such as the countless "hairs" that hang from his body.

The Rav is referring to the spiritual counterpart of the body hair. Not just limbs, but even incidental aspects such as the spiritual counterparts of body hair may be the root of souls.

All the sparks of the individual souls in this world are from these aspects that are within Adam. All of the specific souls and levels of souls that will later enter bodies born from Adam are called "Soul-Roots" and are drawn from these aspects of Adam.

Concerning each and every limb, it is exactly as the sages understood the verse, "Where were you when I founded the earth?" (Job 38:4).

This was discussed at length in Chapter Three, Section 2. The Hebrew words for "where were you" can also be construed to mean "a lifeless lump."

The roots of all the souls that would descend from Adam were implanted within him…

When Adam was only a lifeless lump, G‑d breathed into him a living soul. At that time the roots of all the souls that would descend from Adam were implanted within him.

When Job complains about his lot, G‑d says to him, "Where were you when I founded the earth?" In other words, "If you do not know the root of your soul and where you were in the soul body of Adam when I breathed into him a living soul, then you do not know your own history, and, if this is so, how can you complain about what happens to you?!"

We have already explained that most souls are from Cain and Abel, the sons of Adam.

This statement is problematic, since there are several places in the writings of the Ari himself that seem to contradict it. See, for example, Benei Aharon.

From that time onward, they have since divided among all people ever born. Even though we won't explain this concept in detail now, since this is not the place to do so, we will provide an example of what it means.

For example, suppose that Abel is the aspect of the head of all the souls. Then Abel would be the root of all the individual sparks of human souls that are in the aspect of the head.

This type of kabbalistic conception was discussed in the extended note to Chapter Five, Section 5: Connected Levels of Soul.

Likewise, if our patriarch Abraham was the right arm of Adam, then all the individual sparks that belong to the right arm of Adam are included in Abraham. He will be considered the root of all of them.

It is the same for every limb and hair of the soul of Adam. However, this is not the place for this discussion.

[Commentary by Shabtai Teicher.]