What is the "planned arc" of a series and what does it change?

When you create or edit a series (Library → click the series name, then "Edit" on The plan card), you can write the premise (what the saga is about) and the planned arc — where the story is going, book by book, e.g. "Book 1: the discovery. Book 2: exile. Book 3: the return." Both are sent to the AI while it writes EVERY volume, with instructions to plant seeds for what is coming, never contradict the plan, and not rush ahead of the current book's place in it. That is how early books can quietly set up events that only pay off volumes later. You can refine the premise and arc at any time.