When you see something like "x Arc" preceding the titles of a set of chapters, that means you're reading a select story arc (obviously).

When you simply see "x" (without the word "arc") preceding a set of chapter titles, that is what I refer to as (in this context, as other authors may have different conventions for organization) a sub-arc. The rule here is that all sets of sub-arcs belong to the last mentioned "x arc."

For example, sub-arcs such as "Creation," "Prodigal Wannabes," "Black Rose City Visit," and "Hell on Circuits," all belong to the collective "Hell-Circuit Arc" because of this rule. As a quick recap, all sub-arcs following "x arc" are part of that arc. Once a new "y arc" begins, the following sub-arcs (if any) are part of "y arc," until the beginning of "z arc," and so on. That wasn't much quicker than the first explanation, but you get the idea. Have fun reading