From https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Using_HTML_sections_and_outlines:
The HTML Aside Section Element (<aside>) defines a section that, though related to the main element, doesn't belong to the main flow, like an explanation box or an advertisement. It has its own outline, but doesn't belong to the main one.
From https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Using_HTML_sections_and_outlines: