The Tag Missile was a unique missile that appeared in Twisted Metal. It was removed from the game and its main function does not work, but the missile itself can be accessed via a cheat device, and it still has some use as a weapon. Tag Missiles have a bright green trail and fly straight, rather like a Power Missile. It does very little damage when it hits, being a bit weaker than the Fire Missile.
According to the game code, Tag Missiles were intended to "mark" other cars, making it so that Fire Missiles and Homing Missiles would seek them out without having to acquire targets. This was done away with when it was decided to allow missiles to find their own targets when launched. As such, the missile was largely made irrelevant. If included with a cheat device, they're essentially a useless weapon, with a large number of shots but essentially chip damage and no guidance system.