Usually one codex is valid until a revision is made, so all codexes from the 6th, 5th, 4th and 3rd Editions of ''Warhammer 40,000'' are still valid in the current, 8th 9th Edition, unless a later version has replaced it. There have been instances of codices that are not "standalone". They were known as "mini-dexes". These were usually army lists that were variants of a mainstream 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.