Service Tag

What are Service Tags?
Service tags are tags that consist of one letter then two numbers following and are meant to identify soldiers quickly during battle. They show-up when another soldier "sees" you though 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, are fighting for them, are saving them, etc. 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 of being vulgar, offensive, etc. Here are some of them :

A55, N19, F49, T17

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