Originally Posted by Feisty
I have no idea what "H2/3 styles" or "TOC" are and might get kind of lost...
TOC = Table Of Contents
H2, H3, H4, etc = Levels of headings, defined in as HTML tags (an ePub is really just an HTML document). The lower the number, the bigger the heading. e.g. <h1>Heading</h1> is a big heading.
Sigil has a WYSIWYG (What You See Is What You Get) editor, so it hides this underlying code from you, and you can edit an ePub in a non-techy way, as if you were editing a document in a word processor.