The most important thing to understand about this planner is where your plan lives. Everything you enter is stored in your own browser and nowhere else — no account, no upload. That makes it private and instant, and it also means clearing your browsing data deletes it, and the plan you built on your laptop will not appear on your phone.
What it holds is a book broken into chapters. Each chapter carries a title, a summary, private notes and a word count target, and moves through three states as you work: not started, in progress, and completed. You can keep several books side by side, which is what makes it usable for a series rather than a single title.
You can start empty or from a built-in structure. The novel template lays out a nine-chapter arc from hook through inciting incident, midpoint and dark night to climax and resolution; the how-to template runs problem, foundation, three steps, common mistakes and conclusion. They are scaffolding to argue with, not a formula.
In your browser's local storage on the device you used, and nowhere else. Nothing is transmitted and no account is involved, so the plan is completely private. The trade-off is that it is also unbacked: clearing site data removes it, and it does not follow you to another browser or another machine.
Yes, and that is the intended use for a series. Each book keeps its own chapters and characters, so you can hold the outline for a book you are drafting beside the one you are planning, without the two sets of notes mixing.
There is no correct answer, which is why the target is a field you fill in rather than a number the tool suggests. Chapter length varies enormously by genre, by voice and by what a particular chapter has to do. The useful discipline is internal consistency: readers notice when chapters vary wildly for no reason.