Heading settings define defaults for H1 through H6. Project settings are broad defaults. Book settings can inherit or override them for one book.
Pre-heading silence is the pause before a heading. Post-heading silence is the pause after the heading text and before that heading's section content. Post-subtree silence is the pause after that heading's full section subtree has rendered.
Audio cues are shown as start-count::end-count. Start cues play before the heading text. End cues play after the heading text.
TOC means table of contents. The book icon marks heading levels included in book or ebook output. The headphones icon marks the heading level used for audiobook navigation.
Targets still matter. A heading selected for book TOC should render to book output, and a heading selected for audio TOC should render to audio output.