Assiyah means action, making. For Cordovero, it is the most revealed of the four worlds: "the name testifies to it, for the language of making designates the thickness of things to the highest degree." He places in it everything made from the first point of action, everything the account of creation tells, "down to the point of the abyss," and he includes in it the kelipot, the husks.
Etz Chaim describes this world precisely. It includes the seven firmaments, the highest of which, Aravot, corresponds to the first three Sefirot; the spheres of the seven planets and the twelve constellations; then the earth, with the four elements, fire, air, water and dust. "There matter and the vessels reach completion," at the end of the descent. That is why the husks are fully revealed there, so much that it is called "the world of the husks," although it also contains ten Sefirot of holiness.
Why such a world? Cordovero answers from the Zohar: to subdue the husks, so that the verse may be fulfilled, "and His kingship rules over all." In Assiyah, sparks of holiness clothe themselves in the ten husks, and this is what is called the ten Sefirot of Assiyah. He adds a detail: in the land of Israel, Assiyah does not rule, for there are no husks there that govern.
Massekhet Atzilut populates this world: there are the ofanim, who receive prayers and requests, there are the vital souls of human beings, and Sandalfon reigns over them; above, G-d "gives them light and rules them all." The Ari links Assiyah to the final heh of the Name, to the Nukva and to nefesh, the vital soul; he quotes the formula: "Malkhut nests in the ofan." Cordovero takes up this formula and explains it through the angels called ofanim, who act upon the heavenly spheres.
Etz Chaim adds that the four worlds are made from the sorting of the broken "kings": the best went into Atzilut, the lesser into Beriah, then into Yetzirah, then into Assiyah, and what could not be sorted remained in the husks. Shaar HaKlalim specifies that the sparks left in the husks are raised by prayer and good deeds. The world of action is therefore the one where the most sorting remains. Keep this in mind: it is here, through your deeds, that this sorting is done.
