Passage-level retrieval is the retrieval of individual sections or passages from a document rather than the document as a whole. Modern RAG systems embed and retrieve at the chunk level — the retrievable unit is the passage that answers a sub-query, not the page that contains it.
Passage-level retrieval is the mechanical reason that page-level optimization is insufficient for AI citation. A page optimized for the parent query may contain no passage that cleanly answers any individual sub-query. A page structured around sub-query-answering sections — one question per section, answer-first — produces multiple distinct retrievable units from a single page. The page builds topical authority; the sections earn citations.