RFC 9110 defines 301 as a permanent move where user agents and caches should update stored references. Search engines treat a 301 as the strongest instruction to transfer ranking and backlink equity from the old URL to the new one; Google has confirmed that 301 passes nearly all ranking signals after a stable period of crawling. Historically 301 had a quirk that let clients downgrade the HTTP method from `POST` to `GET` when following, which is why API redirections often prefer the stricter 308 response. Browsers cache 301 responses aggressively and some will reuse the cached redirect without revalidating for days, so reverting a 301 in production means users keep hitting the old target until local caches expire — this is why experienced operators treat 301 as close to irreversible. 301 is the right choice when a page has genuinely moved, a domain has been rebranded, or duplicate URLs are being consolidated onto a canonical. It is the wrong choice for campaign links whose target might change.