Two kinds of direct and returning light: in the governmental order and in the developmental chain.
Eyn Sof encompasses the Sefirot above and below — I am first and I am last (Isaiah 44:6). The cycle runs from Eyn Sof and returns to Eyn Sof.
Op. 16 takes the strongest move yet on cosmic temporality. Eyn Sof is not just the source from which the cosmic structures emerge; He is also the destination to which they return. The cycle of creation runs from Him to Him. The chapter also names the Tzimtzum explicitly for the first time.
Klach cites Isaiah 44:6 — I am first and I am last — as the biblical anchor. The same Eyn Sof that begins the cosmogonic process is the Eyn Sof to which the cycle's completion returns. The arc from cosmic beginning to cosmic end is not a linear movement away from Him followed by a separate movement back; it is a single circular movement of His Will, expressed in two phases.
The two-kinds-of-light doctrine in the chapter's title applies the same idea at the level of cosmic anatomy. Direct light (or yashar) flows from above to below — the source illuminating its receiver. Returning light (or chozer) flows from below to above — the receiver reflecting back. Both lights operate in both the developmental chain (the cosmogonic process) and the governmental order (the operational rhythm). The cosmos is, structurally, a system of forward and returning flows, all of them within Eyn Sof's encompassing presence.
The chapter also names the Tzimtzum explicitly. The Tzimtzum is the concealment of the original simple light of Eyn Sof that filled everything. Within the Tzimtzum, the governmental order proceeds; the goal is the restoration of the original perfection. Op. 16's framing is what the rest of the book operationalises: the Tzimtzum is not a permanent loss; it is the operational space within which the cycle runs back to its source.
The Tzimtzum is given its full operational definition at Op. 24. The cycle's terminus is named at Op. 79 (the Great Day of Judgment) and at Op. 137 (six thousand years culminating in complete repair for ever to eternity). The two-lights doctrine is operationalised at Op. 28 (Inner and Encompassing Light) and at Op. 127 (the Tzelem doctrine of Inner and Encompassing Mental Powers).