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.

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 are some of them:

A23, A20, A55, N19, F49, T17, N64, L17, I17, A00, Z00, P53, W17, P52, P51, M00, J00, G00, A01, B69, I69, T12, T17, E29, 000, D00, P15

There still may be more, but the few above are the tags that have been discovered.