Arich Anpin is the root; the other Partzufim are branches.
A"A is the root of all Atzilut Partzufim. The others are branches. A"A is the first specifiable cosmic level below radla.
Op. 90 opens the Partzuf of Arich Anpin unit (Op. 90–95). After the radla unit's ceiling-of-investigation, Op. 90 establishes the first specifiable cosmic level — A"A. Every Partzuf in Atzilut is a branch of A"A; A"A is therefore the structural-functional root of all operational cosmic governance.
A"A (the Long-Faced One — long of patience) is the operational root of Atzilut. Its name contrasts with Zeir Anpin (Z"A) (Short-Faced — short of patience) in a structurally meaningful way: at the root level, the cosmic government's posture is patient, unmitigated mercy; at the operational level, it is limited, judgment-tempered-by-mercy. The patience-long-shortness opposition is the cosmic-anatomical form of the depth-from-which-the-mercy-comes.
A"A is built of MaH (gematria 45) and BaN (gematria 52) — the matkela-joined cosmic dyad — operating as the male and female aspects (dechura ve-nukva) of A"A itself. This is structurally distinctive: in lower Partzufim, MaH and BaN appear as two distinct partners (male and female); in A"A, they are aspects of one Partzuf, not distinct partners. (Op. 73's degrees-of-union.) The other Atzilut Partzufim — Abba, Imma, Z"A, Nukva — are branches* of A"A in the sense that they emerge from A"A's operational radiation and operate within A"A's structural framework.
Op. 91–95 will unfold A"A's specific anatomical features (Skull and Brain; Three Heads; thirteen Conduits of Mercy). Op. 96 will name the chain AK → Atik Yomin → A"A → Atzilut. The branches-of-A"A claim runs through the rest of the book.