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 current screen. This is used in the Halo 3 Multiplayer.

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.

Dis-Allowed Service Tags

Some certain service tags are not allowed because they are vulgar, offensive, or copyrighted. Here is a complete list of Dis-allowed service tags.

A23, L17, I17, E43, A00, B00, C00, D00, E00, F00, G00, H00, I00, J00, K00, L00, M00, N00, O00, P00, Q00, R00, S00, T00, U00, V00, W00, X00, Y00, Z00, A01, A20, A55, C17, E51, E74, F43, F46, F48, F49, N19, N34, N64, N84, P52, T17, U50, V50, Z17

• When I17 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.
• N64 is the common abbreviation for the Nintendo 64 console. P51, P52, and P53 aren't allowed because they resemble the abbreviations PS1, PS2, and PS3 for Sony PlayStation series consoles.