The Rebbe established a publishing house called “Otzar Hachassidim”—“The Treasure of the Chassidim” to release the writings of the Rebbeim.
The Rebbe designated the collective title for the works of the Rebbeim of Chabad as “Shalsheles Ha’or”—“The Chain of Light.” Thus, the Rebbe writes in his preface to the Rebbe Rashab’s Chanoch Lenaar:
The “Otzar Hachassidim” publishing house is moving toward releasing a compilation known as “Shalsheles Ha’or.” This is a compilation of booklets, each one dedicated to one of the Nesi’im and Rebbeim of Chabad chassidim. The first ring in this chain is the light of the teachings of the Baal Shem Tov, and each link is linked to the next, until the [Previous] Rebbe, my father-in-law, shlita.Each “link” in the chain—i.e., the works of each Rebbe—is referred to as a “heichal,” literally a “chamber” (pl. “heichalos”). There are altogether nine heichalos, one for each of the Rebbeim.
Each heichal in turn contains many “she’arim,” literally “gates” (sing. shaar). Each shaar is devoted to a different type of teaching, e.g., one for Chassidic discourses, another for letters, and yet another for halachic works, and so on. It should be noted that not all of the she’arim for the various heichalos were arranged in the same fashion.
The heichal and shaar is written on the shaar blat (title page) of the book. (This was introduced by the Rebbe, and thus not found in previous printings; for example, see the shaar blat of Pokeach Ivrim here.) For example, see here; this is the shaar blat of a volume of the Previous Rebbe’s holy letters. At the top you will see emblazoned “Library — Otzar Hachassidim — Lubavitch.” This can be found on the top of the shaar blat of all works produced by Otzar Hachassidim.
Then below it, in the top center, it is written “Kovetz Shalsheles Ha’or”—“The compilation of the chain of light,” which can be found on the top of the shaar blat of all works from the Rebbeim (unlike, for example, Reb Hillel Paritcher’s Pelach Harimon—see here). Then below it on the right it says “the eighth heichal,” for the Previous Rebbe was the eighth Rebbe when counting from the Baal Shem Tov, and on the left it is written “second shaar” to denote the section devoted to the Previous Rebbe’s letters.
Based on an article in Oholei Lubavitch, Nissan/Iyar 5755.