Choose 301 — Permanent when the old address is gone for good (renamed page, migrated site, retired campaign page). Search engines transfer the old page's ranking to the new address and update their index. Choose 302 — Temporary when the original address will come back (a seasonal page temporarily pointing to a promo, A/B testing) so search engines keep the original URL indexed. If unsure, use 301 – it is the default. Anything other than 301/302 is saved as 301.