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The element for a thematic grouping of content with a heading — a chapter of a page, one group of a catalog, a step of a form — which becomes a region landmark only when it has an accessible name (aria-labelledby pointing at its heading) and is otherwise just a <div> that means something

The most overused semantic element, because it sounds like it does more than it does. A <section> without a heading is a <div>; with a heading and a label it is a region a screen reader can list. The resources index uses it correctly — one <section aria-labelledby> per kind, so the landmark list reads Books, Courses, Extensions — and the feed groups its months the same way. The test: would it appear in an outline of the page? If not, it is a <div>.