The line, though, is the gift of relative “perfection that God allowed back” into the environment created by the Tzimtzum “in order to (allow the created entities to) perfect all of their imperfections” to a degree. That’s to say, as he goes on to explain, the “perfection that God brought back” was the kind that was “relative to their (i.e., the created entities’) makeup, and not relative to His own His makeup” which would be utter perfection. The latter refers to the presence of God that remains outside of and surrounds the world’s environment (Klallim Rishonim 6).
In other words, there are three environments: 1. There’s God’s own, which is removed from the world’s and is represented by God’s presence surrounding the “space” created by the Tzimtzum; 2. there’s the imperfect and ambiguous environment allowed for by the existence of the (mere) “trace” of God’s presence; and 3. there’s the sort of relative perfection that the world can achieve in the environment created by the “trace” thanks to God having allowed His presence to shine upon the “trace” environment by means of the “line”.
That’s to say that Ramchal apparently sees Ari’s system as a means of depicting the drama of God’s interactions with reality: of God allowing for it, His granting it its challenges and its possibilities, and His standing at a distance from it while yearning for its success.
Put another way and taking great liberties in the process, we’d suggest that Ramchal could be said to see the Kabbalistic system as a depiction of God’s “parenting” of the universe, so to speak; His leaving it on its own in an imperfect but tenable environment that He supplies and provides for; and His enabling it achieve its potential (without granting it that from the outset — like a wise parent who refuses to spoil his child and wants him instead to reach his potential on his own, along with the parent’s encouragement) .
 Note that the eventual revelation of God’s full sovereignty will undo the image of Him as a loving, hands-off parent in that all prove to have been in God’s hands from the first, but that’s beside the present point. See Adir Bamarom p. 266 for more on the connections between the Tzimtzum, trace, and line.
(c) 2011 Rabbi Yaakov Feldman
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Rabbi Yaakov Feldman has also translated and commented upon “The Path of the Just” and “The Duties of the Heart” (Jason Aronson Publishers).