Note that the basic Substack format is not the only option. Some writers redirect to their own web page, either directly or by "click to continue reading" where you can write in a real web page, with any format you want - sidebars for definitions, chapters, whatever. The basic substack page format is defective in many ways and they will not fix it, and you get bashed for comments. For instance, you cannot underline a book title. Any link not embedded gets blow up into a big picture.
Note that the basic Substack format is not the only option. Some writers redirect to their own web page, either directly or by "click to continue reading" where you can write in a real web page, with any format you want - sidebars for definitions, chapters, whatever. The basic substack page format is defective in many ways and they will not fix it, and you get bashed for comments. For instance, you cannot underline a book title. Any link not embedded gets blow up into a big picture.