To apply this to the subject of our present discussion: The lights are preparations on the level of thought for all the different created realms and beings and the entire governmental order through which they are run. However, for these to come into actual existence is a further stage, because thinking about something does not necessarily make it actually happen. If so, we need to identify a separate root through which they actually come into being.
The particular root involved here is the letters. Accordingly, in order for the supernal lights to produce an actual effect, they must enter into the category of letters. The letters consititute a single root produced through a separate radiation. Everything prepared in the Sefirot on the level of thought must enter this level and pass through this root in order to come into actual being.