Daat expands through the whole of Zeir Anpin, while Chochmah and Binah do not.
Daat is the bridging Mochin — the offspring of Abba-Imma engrafting via Chochmah and Binah. Only Daat spreads through the whole of Z"A.
Op. 124 opens the Daat of Z"A unit (Op. 124–126) by specifying Daat's distinctive role. Among the three Mental Powers (Chochmah, Binah, Daat), only Daat spreads through the whole of Z"A's body. The chapter names what makes Daat operationally distinctive.
Daat is the offspring of Abba and Imma's engrafting via Chochmah and Binah. Abba contributes Chochmah's whole-revelation; Imma contributes Binah's detail-revelation; together they engraft into Z"A, and the offspring is Daat.
Daat is structurally between: between Chochmah and Binah (the parents); between Z"A and Nukva (its 5/5 distribution per Op. 125 will reveal); between the upper Mochin and the body of Z"A (its spread through the body).
The threefold bridging role makes Daat the operationally distinctive Mochin. Without Daat, there is no intrinsic function of Z"A — Op. 124's strongest claim. Z"A's CHaDaR governance, Z"A's coupling with Nukva, Z"A's engrafting outputs (Op. 117) — all of this depends on Daat. The Mental Powers Chochmah and Binah are present in Z"A's Mochin region but do not spread; only Daat does.