Emblems
From Halopedia, the Halo wiki
In Halo 2, when a Character Profile is created, there is an option to make an Emblem for your character. These emblems are only used in Multiplayer modes and online play, allowing other players to easily identify you in battle. Also, in the gametype Territories, your emblem appears on a flag inside the area of a territory if you take it. The color and background of the emblems are usually determined by the primary and secondary armor colors. On a player model, these emblems are seen glowing on his/her back and both shoulders.
Halo 3 Emblems
The emblems are very similar in Halo 3, except there are more choices. Players can choose from any of the Halo 2 emblems (except Sergeant) as well as some new ones. Also, players can choose a background for their emblems. However emblems do not represent you as much as they did in Halo 2. The new representative is a letter and two numbers (i.e.: D30)It is also noted that (for Spartans) the emblems only appear on the right shoulder. For Elite player models, they appear on the right shoulder and upper back. In Halo 2, they were on both shoulders and the back of the character for both player models.
Considering 80 primary emblem designs, 80 possible "toggles" of those designs, 48 possible emblem backgrounds, and 30 possible colors for each of the above, there is a combined total of 8,294,200,000 Halo 3 emblem possibilities. That's over 8 billion.
