Three levels, fixed
The Books module gives documentation a shape that ordinary pages do not have:
- A book is a whole body of documentation — "User Documentation", "Developer Documentation".
- A unit is a chapter or topic within it — "Getting started", "Forms and submissions".
- A page is a single article inside a unit.
The depth is fixed at three. You cannot nest a unit inside a unit. That is a constraint, and it is the point: it stops documentation growing a private taxonomy nobody else can navigate.
Why not just use pages?
Pages nest to any depth and are perfect for a site's general structure. Documentation has a property pages do not model well: reading order.
Because a book knows its units are ordered and each unit knows its pages are ordered, the platform can offer things that would otherwise be hand-maintained — a chapter list, a sidebar tree, previous and next links, a page count, and a whole-book PDF export.
What this costs you
The trade is flexibility. A book page cannot sit outside a unit, and a unit cannot sit outside a book. If a piece of content genuinely does not belong to a chapter, it probably wants to be a page rather than a book page.
Choosing between them
Use a book when the content is meant to be worked through, has a natural order, and benefits from being browsed as a set: onboarding guides, developer references, training material.
Use pages when items are read standalone and arrive from search or a link: news, marketing, policies, landing pages.
A useful test: if "what comes next?" is a meaningful question for the reader, it is a book.