Details of the Residue that filled the Empty Space.
After the Tzimtzum, the contracted space is not empty in itself — it contains the Reshimu (residue), the trace of the original simple light, on which all later cosmogony will operate.
The Tzimtzum opens a chalal — an empty space — within Eyn Sof's encompassing limitlessness. Op. 26 specifies what's in that empty space. The answer: a residue (Reshimu), the trace of the simple light that filled the place before the contraction. The chalal is empty only in relation to Eyn Sof's encompassing limitlessness; in itself, it is the receptive matrix of all later cosmogony.
The Reshimu is what makes the post-Tzimtzum cosmos possible at the receiving end. When the Kav (Op. 27) enters the chalal carrying Eyn Sof's perfect action, it has to be received by something. The Reshimu is that something. Without the Reshimu, the contracted space would be a true vacuum and the Kav would have nothing to act on. With the Reshimu, the chalal is a receptive matrix, ready to be illuminated and structured.
This sets up the Line-and-Residue pair that becomes load-bearing for the rest of the book. Line is the active partner; Residue is the receptive partner; their interaction is what produces the Sefirot, the Partzufim, all the subsequent cosmogonic structures. By Op. 64, this same pair is renamed MaH (gematria 45) and BaN (gematria 52) — the same cosmogonic dynamic, sharpened operationally for the Repair unit.
The chapter's empty only in relation to phrasing is also worth holding. The chalal is empty in contrast to Eyn Sof's surrounding limitlessness — but Eyn Sof has not gone anywhere. The chalal is willfully restricted, not abandoned. He encompasses it from outside; the Reshimu sits inside it; the Kav will pierce it. None of these motions weakens His sovereignty.