Baal HaSulam, Beit Shaar HaKavanot (The Gateway of Intentions), Item 87: And know that the complete evil that becomes clarified, in which there is nothing good [It is the scrutiny of the three external Klipot (shells) which will only be corrected at the end of correction (Gmar Tikkun)], but the fourth Klipa is not fully clarified, and that is why it stays in the form of “garment” and “skin.”
This means that there is something good in it, but it is not yet scrutinized, and that is why it is called Klipa (shell). However, still, it is not a Klipa like the three impure Klipot which clearly carry no good in them, and hence, we cannot correct them until the final correction.
However, we can scrutinize the fourth Klipa, Klipat Noga, and we need to work with it. It is the broken Kelim (vessels) that fell into the worlds of BYA (Beria, Yetzira, Assiya) where good and evil are mixed together. We are clarifying them in particular in the Klipat Noga called the middle third of the Sefira Tifferet.
This is the only place of our freedom of will. Above it, there is sanctity; there is nothing to scrutinize there. Below, there is complete impurity, the Klipa; there is nothing to sort out there either. We can only work in the Klipat Noga where there is still a mixture of good and evil.
It turns out that it is precisely the place where we have to constantly reside: the place of confusion and doubt, where everything is mixed, unclear, and dark. This is where we make clarifications; it is precisely the place of our work.
From the 3rd part of the Daily Kabbalah Lesson 12/20/10, Beit Shaar HaKavanot