From Halopedia, the Halo wiki
Developer Mode, or "Devmode", is a utility used primarily for Halo Custom Edition. It allows commands to be entered during gameplay using a console and is commonly used to test user-created maps.
To create a Devmode shortcut, find a shortcut to Halo Custom Edition and copy it. Then, right click the new shortcut's icon, and select Properties. A text field labeled "Target" will have the value "C:\Program Files\Microsoft Games\Halo Custom Edition\haloce.exe"; change this to read "C:\Program Files\Microsoft Games\Halo Custom Edition\haloce.exe" -console -devmode.
To load Halo Custom Edition with Devmode enabled, simply open the new shortcut. Do note however that shortcuts with Devmode enabled will prevent the user from joining internet and LAN servers, and removes one's ability to create an internet game. the user will only be able to create a LAN game.
List of commands
Commands marked with "<#>" require a 1 (for activating) or a 0 (for deactivating) in place of the symbol.[alternate 1]
The following is a simple list of useful commands. For a list of commands, simply press the Tab button with the console opened.
Alldev and Devtrainer
Alldev is a devmode program that works for Halo: PC and for Halo: Custom Edition multiplayer. Basically, it allows server hosts to cheat. Non-hosts can use the lesser cheats, but cheats such as invincibility can only be achieved by the host. Often, people will host servers with Alldev for the sole purpose of having fun with cheats on.
Devtrainer, or Developer Trainer, is another devmode program which also works for Halo: CE. Instead of allowing the user to use commands on the console, it has a simple list of command codes built into it. Basic cheats such as super jump and invincibility can be switched on with the press of a button. One of DevTrainer's greatest features is its built-in ability to freeze time, allowing the player to stop all slow-moving projectiles, grenades, AI-controlled characters, and vehicles. It does not allow the player to freeze other players in place, unless they happen to be driving a vehicle.