Rabbi
Moshe Chaim Luzzatto ("RaMChaL"):
138 Openings
of Wisdom
Opening 90
Arich Anpin is the root; the other Partzufim are branches.
Arich Anpin is the first Partzuf in Atzilut and is built of MaH and BaN. Arich Anpin is the root of all the rest of Atzilut, for all the other Partzufim are literally branches of Arich Anpin, which executes its functions through them.
Having discussed Atik, we must now discuss the Parzuf that comes after it - Arich Anpin.
Arich Anpin is the first Partzuf in Atzilut... For the place where the ten Sefirot of any given world start is called the beginning (ראשית, reishit) of that world. In the case of Atzilut this is none other than Arich Anpin, which is Keter of Atzilut. Atik, on the other hand, is not actually a part of Atzilut itself but is considered related to Atzilut inasmuch as Atik exists for the sake of Atzilut and is clothed in and directs it.
...and is built of MaH and BaN. For all of Atzilut is built of MaH and BaN. In the case of Arich Anpin, this MaH and BaN are its male and female aspects.
Arich Anpin is the root of all the rest of Atzilut... The difference between Arich Anpin and the other Partzufim is that each of the other Partzufim is a separate branch. Even though in terms of the way they develop they emerge from one another, the truth is that they are not branches of one another. Rather, they are all branches of Arich Anpin. Accordingly, Arich Anpin cannot be considered in the same category as the other Partzufim of Atzilut since Arich Anpin is the root while the others are branches of Arich. However, when speaking of Atzilut as a whole with its root and branches, we also include Arich Anpin. Rabbi Shimon bar Yochai alludes to this mystery in his words in the Idra Zuta (288a): "Only the Lamp exists".
...for all the other Partzufim are literally branches of Arich Anpin... They are not branches of Arich Anpin merely in the way in which they seem to be branches of one another inasmuch as they emerge from one another in the developmental chain. In relation to Arich Anpin, the other Partzufim are actual branches.
...which executes its functions through them. For the entire governmental order is rooted in Arich Anpin, except that in order for it to be revealed, it divides into the four Partzufim that clothe Arich. These are called "the Name of Atik" (Idra Zuta, ibid.). For this reason Arich Anpin extends the length of the entire world, while all the other four Partzufim clothe Arich - Abba and Imma to the navel, Zeir Anpin and the Nukva from the navel and below. Accordingly, all of them are only garments on Arich Anpin. Each garment clothes only one part, but together all of them clothe Arich Anpin completely.