Changes

Usually one codex is valid until a revision is made, so all codexes from 4th and 3rd editions of Warhammer 40,000 are still valid in the current, 5th edition, unless a later version has replaced it. There have been instances of codexes that are not "stand-alone". They were known as "mini-dexes". These were usually army lists that were variants of a main stream army. An example would be the early 3rd edition Codex: Catachans, which required the main Codex: Imperial Guard to use. Mini-dexes are more or less extinct nowadays. The only mini-dex still in use the Space Wolves one, which uses some of the regular Space Marine Codex rules. However, the Space Wolves are getting a new codex in the next six months. After this, there will never again be a mini-dex.