The Line shines within the Sefirot through a garb.
The Kav does not shine on the Sefirot directly; it shines through a garb (levush). The garb-doctrine is the cosmogonic seed of clothing (hitlabshut) that runs through the second half of the book.
Op. 28 introduced the two-light structure. Op. 29 adds two more elements: a Vessel (kli) and Mochin (mental powers, the garb of the Line). The chapter completes the Sefirah's three-aspect anatomy — and plants the garb terminology that will become hitlabshut (clothing) at the Partzuf level.
A Sefirah, in its operational anatomy, has three aspects: a Vessel (the structural form), Mochin (the mental powers — the garb through which the Kav shines), and Inner/Encompassing Light (the dual illumination from Op. 28).
The garb idea is structurally important. The Kav's light is too direct, too bright, to enter the Sefirah's vessel without an intermediating layer. The Mochin are that layer. They wrap the Kav, mediate it, and present it to the vessel in a form the vessel can receive. This is the cosmogonic seed of hitlabshut — the clothing relation by which one cosmic level transmits its influence through another.
When the second half of the book treats Atik Yomin clothing in Arich Anpin (Op. 96), or Abba/Imma's Mental Powers spreading through Zeir Anpin (Z"A) (Op. 116), or the general rule that every Partzuf-pair operates in two modes — chain of development and clothing (Op. 109) — every one of these is a development of the garb-doctrine planted here.
Op. 70's formal Partzuf definition treats measure of thickness (the inner/outer relation, KBD-within-Chesed, Gevurah, Tiferet (CGT)-within-Netzach, Hod, Yesod (NHY)) as one of three architectural features. The clothing doctrine generalises across the named-Partzuf walk, becoming the rule that distinguishes upper-from-lower-Partzuf relationships throughout the cosmic government.