Service Tag

A Service Tag is a tag that consists of one letter and two numbers, meant to identify soldiers quickly during battle. They show-up when another soldier "sees" you through a wall or if their cross-hair is not pointed directly on you, but they still see you on their screen. This is used in the Halo 3 Multiplayer and in the Halo 3: ODST Firefight.

Advantages
It is good to set your service tag identical to your profile name, so allies can know which of their team-mates are with them. Also, if you cannot directly center a soldier, you are still able to catch their service tag.

Prohibited Service Tags
Some service tags are not allowed because they are vulgar, offensive, or copyrighted. Here is a complete list of prohibited service tags.


 * A23
 * A55
 * L17
 * I17
 * E43
 * Several service tags ending in -00
 * A01
 * A20
 * A55
 * C17
 * E51
 * E74
 * F43
 * F46
 * F48
 * F49
 * I69 (permitted until a patch was released)
 * N19
 * N34
 * N64 -resembles "N64"
 * N84
 * P52 -resembles "PS2"
 * P53 -resembles "PS3"
 * T17
 * U50
 * V50
 * Z17

Trivia

 * When I17 (resembling 117) is entered as a service tag it will say "this service tag is currently reserved by the UNSC," likely due to the fact that it resembles John-117's tag.
 * Some of the above service tags (I17, for instance) actually can be used. It's pure luck, however, as you may be given that service tag at random when you first play Halo 3 on an Xbox 360 profile.
 * Some sevice tags that resemble Playstation or Nintendo products, like N35 (NES), P59 (PSP), W11 (Wii), and P51 (PS1 or Playstation 1) are not banned and can be used.
 * Several people use the tag O07 (resembling 007 aka James Bond).
 * When people die, the servie tag is replaced by a red cross, indicating that the player is dead.