Some manuscript content belongs only in the book output. Other content belongs only in the spoken audio path. This lets an author keep a visual passage and a spoken equivalent close together without pretending they are the same thing.
Use book-only content for material that depends on the page, such as diagrams, some math, or visual layout. Use audio-only content for spoken clarification, alternate wording, or sound-specific transitions.
Target-specific duplication can be intentional. Review nearby book-only and audio-only blocks before deleting text that appears repeated.