Diagram

One query, two rankings, one list

The query goes to BM25 and to vector search. Each returns a ranked list. Fusion merges them into one list. Query "server down" BM25 scores shared words rank 1, 2, 3 Vector search scores closeness of meaning rank 1, 2, 3 Fusion RRF, k=60 1 list
Both searches run on the same query. Fusion reads only the rank of each result, so a BM25 score and a cosine score never have to be put on one scale. In XERJ both run inside one index and one request.