There are just a couple of points to underscore about all this before we move on. There’s this statement in Petach 28: This co-existence of two modes of Divine governance: that of the actual and imperfect one of the trace, and that of the potential and perfect one of the line is (i.e., touches upon the idea of) the “internal” and “external” lights of the Sephirot which is (i.e., also touches on) the mystical notion of the vessel.
As Ramchal explains in his comments here, the “internal” and “external” lights refer to God’s “encompassing light” (or, “presence”) as opposed to His “permeating light” (or, “presence”) . God unto Himself, outside of the space created by the Tzimtzum, is termed His “encompassing light”, in that it hovers over and around the place (and all of existence); God’s manifest presence within the place and existence is termed His “permeating light”, in that it’s situated within the space.
Both of these manifestations of God’s presence function as a “light” (or, “as the essential content”) behind and within reality, which itself functions as a “vessel” (or, receptacle”) for that light. This is analogous to the relationship between the “body” and “soul”, in that the body likewise functions as the soul’s “vessel” .
The final though vitally important idea to consider about the line itself is that it functions as the central-core to the Sephirot and Partzufim which over-cover them and serves as its tools.
As Ramchal termed it in Petach 29: The Sephirot are what came forth from the mystical space that occurred at the time of the Tzimtzum. But each Sephira is a part of the trace, for within each Sephira lies an inner core that came about by the line of the Ein Sof entering it. This line shines within the inner core of each Sephira with the mystical implications of a soul .… which is, i.e., which functions as, the line’s “garment”.
 See Zohar 3, 109b.
 There are also two sorts of souls (five actually): an “encompassing” over-soul and a “permeating” animating soul. But that’s beyond the present discussion. See Ramchal’s comments to Petachim 28 and 29 for some explanation of this dynamic.
(c) 2011 Rabbi Yaakov Feldman
Feel free to contact me at firstname.lastname@example.org
AT LONG LAST! Rabbi Feldman’s translation of Maimonides’ “Eight Chapters” is available here at a discount.
You can still purchase a copy of Rabbi Feldman’s translation of “The Gates of Repentance” here at a discount as well.
Rabbi Yaakov Feldman has also translated and commented upon “The Path of the Just” and “The Duties of the Heart” (Jason Aronson Publishers).