The Anti-Air Tank is a tank unit that is used to attack air units, similar to the anti-air soldier and anti-air turret. It does massive damage against air units, giving players a huge advantage against an enemy's air force. However the tanks does not attack land units. This weakness allows soldiers to attack first and an air force can follow. To counter this weakness, guard it with heavy/light tanks to prevent this from happening.
Ita could be possible that the Anti-Air Tank is simply a Light Tank with a AA laucher/gun rather than a general purpose turret.