What This Says About the Ranker

Cupertino's BM25F weights combined with the RRF source authority produce a strong preference for the modern Swift form. The behaviour is not accidental:

1. The Swift form is the framework-root for its concept (e.g., foundation/url is a struct page, not just a method on a class). Framework-root pages have a small BM25F boost (framework=2.0 weight) and are favoured by the dedicated kind-multiplier in Search.Index.Search.swift.
2. The Swift form's title (e.g., "URL") matches the query exactly; the Obj-C form's title (e.g., "NSURL") matches the query as a prefix-of-token only, costing BM25F. With title=10.0 weight, this is the dominant signal.
3. Modern Apple-docs pages have richer content (longer overview prose, more code examples). The Obj-C reference pages are largely stubs with declaration-only content. Length normalisation in BM25F penalises stubs.
4. The Apple-docs source weight in RRF (apple-docs = 3.0) treats both forms equally as far as fusion goes; the within-source BM25F decides the order, and within-source the above three factors all favour Swift.

The result is empirically what an AI coding agent in 2026 needs: searching "URLSession" returns the Swift foundation/urlsession value-type page at rank 1, with foundation/nsurlsession (if present at all) buried at rank 2-8.