The Line divides into the Inner and Encompassing Light of the Sefirot.
The Kav, on entering each Sefirah, divides into Inner Light (Or Penimi — within the Sefirah) and Encompassing Light (Or Makif — surrounding it).
Op. 27 said the Kav governs the Residue. Op. 28 says how. Within each Sefirah, the Kav splits into two distinct lights — inner and encompassing. The two-light structure is the structural anatomy of every Sefirah and, later, every Partzuf in the rest of the book.
The Inner Light is what fills the Sefirah from within — its operational interior. The Encompassing Light is what surrounds the Sefirah from outside — its operational context, the frame within which it acts. Both are the Kav's light, divided.
Why the division? Because a finite vessel can only contain a finite portion of an infinite source. The Inner Light is what the Sefirah's vessel can contain; the Encompassing Light is what surrounds it without entering — present, available, but not absorbed. The vessel's operational identity comes from the Inner Light; its operational reach comes from the Encompassing Light.
The two-light structure becomes load-bearing. Every Sefirah has it; every Partzuf has it (formalised at Op. 70 as measure of thickness — the inner/outer relation). The Mochin doctrine of the Zeir Anpin (Z"A) unit treats Inner and Encompassing Mental Powers as both required for complete governance (Op. 127, the Tzelem). The same structural logic, applied at every operational level.
Op. 29 will add the Vessel and Mochin dimensions, completing a three-aspect picture: Vessel + Mochin + Inner/Encompassing Light. Op. 70's formal Partzuf definition presupposes the two-light structure as one of three architectural features. And Op. 127's Tzelem doctrine makes the Inner/Encompassing distinction operational at the Mental Powers level.