If you are looking for gameplay features related to the Codex system itself:

: Accessing the Doom Slayer's in-game computer allows you to unlock full versions of original games; for example, entering the code FLYNNTAGGART unlocks DOOM II .

: Finding physical Codex pages hidden throughout levels increases your total score and provides 100% mission completion.

The specific entry for describes the expansion of the Argenta empire using the technology and magic of the Maykrs. A key feature of this lore segment is the reveal of the Argenta's first spaceflight , where they used Maykr "machines and enchantment" to travel across the Empyrean Void to spread the Khan Maykr’s word and eventually discover their many enemies.

: Collecting certain Codex entries or completing related challenges can reward players with Sentinel Crystals to upgrade health, armor, or ammo capacity.

In , the Codex serves as a mobile database that provides lore and technical details on the game's world.