The aside element represents a section of a page that consists of
content that is tangentially related to the content around the aside
element, and which could be considered separate from that content.
Such sections are often represented as sidebars in printed typography.
The element can be used for typographical effects like pull quotes or
sidebars, for advertising, for groups of nav elements, and for other
content that is considered separate from the main content of the page.
It’s not appropriate to use the aside element just for parentheticals,
since those are part of the main flow of the document.