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.