aside

The element for content related to what is around it but separable from it — a sidebar, a pull quote, a related-links box, a table of contents beside an article — announced as a complementary landmark when it sits at the top level

The test is whether the page still makes sense with it removed: a table of contents, yes; the article’s second paragraph, no. Inside an <article> it is a tangent (a callout, a pull quote) and not a landmark; at the page level it is the complementary region. This site’s resource pages put their table of contents in one beside the <article>, which is the textbook case, and the shell’s sidebar is a <nav>, not an aside, because it navigates.