A Clan Tag is a four character (maximum) prefix attached to a player's screen name or gamertag. It appears in closed square brackets before the player's username, as "[Clan] Example".
It is a feature used in games from Call of Duty 4: Modern Warfare on to distinguish a player as part of a clan - a group of players. Clan tags are censored on Xbox LIVE for any profanity that may seem offensive, however on PSN any clan tag can be chosen, with every game since Call of Duty: Black Ops as an exception.
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 unlock their customizable clan tag when they reach level 11 - if prestiging, players do not have to wait until level 11 again; the clan tag is unlocked from level 1 of Prestige.
Previously, clan tags were gold-colored if the user had purchased the Call of Duty: World at War Limited Collector's Edition, or linked their account to the Call of Duty Website.
Unique Clan Tags
Treyarch put in twenty-two unique clan tags because of the wait for patching glitches on the PlayStation 3, these tags therefore only worked on the PS3. The clan tags no longer work, unless the player uses hacks.
- [....] - Created a moving ball across the clan tag.
- [RAIN] - Created rainbow colors.
- [****] - Created a moving plus sign.
- [MOVE] - Made the PSN name move.
- [CYLN] - Made a red line move across the PSN name.
- [CYCL] - Made the PSN name cycle colors.
- [ad]- Made the PSN name blood color.
- Gold - When buying a limited edition copy of Call of Duty: World at War players will get a code to make their clan tag gold when activated. The PC version does not get a code, as merely installing the game will add it.
These special Clan Tags have since never returned throughout the remainder of the entire Call of Duty franchise.
Call of Duty: Modern Warfare 2
The clan tag returns in Call of Duty: Modern Warfare 2, exactly the same as it was in Call of Duty 4. The clan tag feature is not available on the PC version of Call of Duty: Modern Warfare 2, most likely due to the fact that changing usernames has no limitation on Steam and thus clan tags can be added in the actual username of the player.
Call of Duty: Black Ops
Clan tag carved into Commando
Clan tags return in Call of Duty: Black Ops. While mostly similar to previous installments, the clan tag can also be carved into various areas of weapons, costing 
1000 per weapon. The ability to carve your clan tag into the weapon(s) is unlocked at level 19. Once the player has bought the ability, one doesn't have to buy it again after each use for the specific gun, unless the player enters Prestige Mode.
Treyarch has added the option to color the clan tag, but the feature is only available after Prestige 13. The colors available are red, green, yellow, blue, cyan and magenta. Each color costs 250.
Formerly, a glitch existed where the player could get a colored clan tag at any prestige. However, it has been patched.
Call of Duty: Modern Warfare 3
The clan tag returns in Call of Duty: Modern Warfare 3, exactly the same as it was in Call of Duty 4: Modern Warfare and Call of Duty: Modern Warfare 2. The clan tag feature is not available on the PC version of Call of Duty: Modern Warfare 3. Clan tags can be colored gold when a player is in an Elite clan that's level 10 or higher.
Call of Duty: Black Ops II
Clan tags re-appear in Call of Duty: Black Ops II similarly to how they did in Call of Duty: Black Ops. However, they can only be colored gold, which is done by linking the account with Call of Duty ELITE.
Certain clan tags that may seem profane or offensive are generally blocked when attempting to save them.
As in Call of Duty: Black Ops, the player can have their clan tag carved onto the weapon they are holding. Entering the first Weapon Prestige will unlock the option for that particular weapon permanently.
Call of Duty: Ghosts
The clan tag remains the same as with most of the previous titles; players can put a four-character title next to their name, which will be applied and appear in all of their Create-a-Soldiers' title card. While in a Clan, players have the option to don that Clan's tag, which will then appear in [ ] brackets. A Clan that reaches level 11 will be rewarded with gold Clan Tags, and red Tags when it reaches the level of 25.
Call of Duty: Advanced Warfare
The Clan Tag returns once again in Call of Duty: Advanced Warfare. Much like previous installments, any tag is allowed except profane ones or company-based ones.
The tag is the standard white/grey colour until the player reaches Clan level 11, where like previous installments, it becomes gold in colour. If the player's clan reaches max level, it becomes red, like in Call of Duty: Ghosts.
Note: In the PC version of Call of Duty: Advanced Warfare multiplayer, a clan tag is customizeable but can't be displayed whatsoever.
Call of Duty: Black Ops III
The Clan Tag returns in Call of Duty: Black Ops III. Like in Black Ops II, the player can also have their Clan Tag written on their weapon, though this time it is shown on an LED display on the weapon (similar to the Kill Counter) rather than carved onto it. This option is available after first weapon prestige.
Trivia
- In Call of Duty: Modern Warfare 2, clan tags go away if the player plays split-screen on Xbox 360 or PlayStation 3. Because of this, players have to remake it when one plays on Xbox Live or PlayStation Network.
- When playing Nazi Zombies or Co-op in Call of Duty: World at War, players cannot change clan tag while in the lobby, unlike in Multiplayer.
- Clan tags that are in certain games restricted to game developers and possibly bots in Combat Training include [IW], [COD] and [3arc]. These cannot be used, as it produces an error message when entered.
- In Call of Duty: Black Ops, there is a counter that tells players how many Clan Tags were purchased globally.
- On the Wii version of Call of Duty: Black Ops, the ability to carve the player's clan tag into a weapon is not present.
- In the game mode Infected in Call of Duty: Modern Warfare 3, if a player gets infected, leaves the game, changes their Clan Tag, and rejoins, they will be on the Survivors side once more.