The government of the worlds is implemented through the letters.
The Sefirot are thought-level preparations. For thought to become actuality, it must enter the category of letters — a separate root, the executive faculty.
Op. 14–17 told you what the cosmic configuration is. Op. 18 opens a new section unit (Letters and Names, Op. 18–23) asking how the configuration becomes operational. The answer introduces a distinction that runs through the rest of the book: thought (the Sefirot as configured) and letters (the executive instrument). Without letters, nothing comes from thought into actual existence.
The analogy Klach uses is the human one. A thought in your mind cannot reach another person until it is cast in language. The thought is real, but it is interior; until it becomes spoken or written, it has no operational effect. The same is true at the cosmic scale. The Sefirot, as configured, are interior — preparations on the level of thought. For the thought to act — for actual creation to come into being, for governance to reach the lower realms — the configuration must enter the category of letters.
Letters are a separate root, produced by a separate radiation. They are not the same kind of thing as the Sefirot; they are the executive faculty by which thought becomes act. The biblical anchor is Psalms 33:6: and through the word of God the heavens were made. The heavens are made by speech — and speech is letters.
This sets up a structural feature of the cosmic government that becomes important much later. Adam Kadmon (the first ordered emanation, named at Op. 31) is the thought; the letters are how the thought speaks itself into being. The cosmic plan is therefore both a configuration of thought and a system of speech-acts; the two together produce actuality.
Op. 19–23 will unfold the letters doctrine: 22 letters as the alphabet of creation; combinations of CHaDaR (Kindness/Judgment/Mercy); the four expansions of the Four-Letter Name; the holy Names; the whole Torah is the Names of the Holy One blessed be He (Op. 23). The thought-vs.-action distinction Op. 18 introduces is the structural ground of every later operational claim about how the cosmic government acts.