Cohere Rerank is a hosted cross-encoder scoring service that sits after a first-stage retriever in a RAG pipeline. It takes a query and a list of candidate documents pulled from a vector store, BM25 index, or hybrid search system, and returns a relevance score for each candidate by running the query and the document jointly through a transformer, rather than comparing precomputed embeddings. The trade-off is architectural: reranking is more accurate than bi-encoder similarity