Another change in Halo 3 from Halo 2 is how players in multiplayer games are identified. Rather than full gamertags, shorter callsigns are used, although it would appear that player badges are also retained.

This alpha footage shows short callsigns for some players, such as P13 above (who is dead) but longer names for others (such as HALO014 next to him). The distinction may be between enemies and friendlies, or perhaps the build shown does not enforce such short callsigns on all players.