Clan Tag

A clan tag is a four character (maximum) prefix attached to a player's screen name. It is a feature used in games from Call of Duty 4: Modern Warfare on to distinguish a player as part of a clan. Clan tags are censored on Xbox LIVE for any profanity that may seem offensive.

Call of Duty 4: Modern Warfare
Clan tags make their debut in Call of Duty 4: Modern Warfare. The clan tag option can be accessed in the Barracks.

Call of Duty: World at War
In Call of Duty: World at War, Players get a customizable clan tag when they reach level 11. This clan tag is Gold if the player purchased the Call of Duty: World at War Limited Collectors Edition or linked their account to the Call of Duty website.

Special Clan Tags
After the release of the first map pack, Treyarch put in codes to change the color of your clan tag. These codes are:

Green - [GRN]

Yellow - [YELW]

Blue - [BLUE]

Light Blue - [CYAN]

Red - [RED]

Capitalization does not affect color change.

Unique Clan Tags

Treyarch put in twenty-two unique clan tags because of the wait for patching glitches on the Playstation 3. Also, it currently only works on the Playstation 3. A few clan tags have been found, and the rest are still being searched for. NOTE: These Clan Tags no longer work.

[....] - Creates a moving ball.

[RAIN] - Creates rainbow colors.

[****] - Creates a moving plus sign.

[MOVE] - Makes the PSN name move.

[CYLN] - Makes a red line move across the PSN name.

[CYCL] - Makes the PSN name cycle colors.

[KILL]- Makes your user name blood color.

Call of Duty: Modern Warfare 2
Clan Tags return in Call of Duty: Modern Warfare 2. They are pretty much the same as in Call of Duty 4: Modern Warfare. Due to the color coding being the doing of Treyarch, they have not returned in Modern Warfare 2.

Call of Duty: Black Ops
Clan Tags will be returning in Call of Duty: Black Ops. While mostly similar to previous installments, the clan tag will also be carved into various areas of weapons, although this is only confirmed on the Commando.

Trivia

 * In Call of Duty: Modern Warfare 2, clan tag goes away if you play split-screen on Xbox 360 or PlayStation 3. Because of this, players have to remake it when you play on Xbox Live or PlayStation Network.
 * When playing Nazi Zombies or Co-op in World at War, player cannot change your clan tag while in the lobby, unlike in Multiplayer though players are still playing online.
 * In Call of Duty: Modern Warfare 2, the player can come across a glitch where players can use the official Infinity Ward clan tag [IW], but when people normally use it a message will stop you from using it and only game developers can use the Clan Tag but if player adds a " then the message will cancel out and you will be able to use the IW clantag, however this only works in game, and player that uses [IW] clan tag can be reported if found by IW staff for doing such an act. In World at War [COD] clan tag is banned.
 * World at War is a lot more strict when it comes to Clan tags since there is a large amount of letters and figures that can't be used, but in Call of Duty: Modern Warfare 2 players can use twice as many letters and numerals.
 * The Clan Tag feature is not available on the PC version of Call of Duty: Modern Warfare 2.