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The heading levels (1-6) are often considered a way of describing and determining the “importance” of a heading, with h1 being the most important. This might be reflected in the visual appearance of headings—higher-level headings typically appear as bigger and bolder text than lower-level headings.

But you can't just put an “important” heading anywhere. Its heading level relates to how much content it refers to, and that has to do with structure.