The icon.
"Formidable on the battlefield, but the aircon is an optional extra"
Heavy Tank is a very rare unlockable 
vehicle. It comes equipped with a 
cannon and a 
machine gun. Although available from level 7, deploying a heavy tank costs 6 
Deployment Points, which requires at least 2 more 
Scouts to be recruited to field a vehicle. By comparison, 3 
Light Tanks can be deployed instead, or just 2 without reinforcement boosts.
To get a vehicle faster, one can specialise and level up 
Pilots, improving the odds to be offered one by 25% each new level.
Deployment
Deployment cost illustrated.
The only video showing a Heavy Tank in combat.
The image to the right was taken from the previous video. Notice that the heavy tank seems to be using 6 Reinforcement Points to deploy. The army in this image is using all 8 of it's Deployment Points, but it does not deploy any troops.
8 Reinforcements - 2 the Light Tank = 6 the Heavy Tank.
If this is true then an army without at least 2 Scouts can not deploy a heavy tank. But even then enemy can reduce deployment bonus by sending 
Spies and/or utilizing 
Reverse Attack. That explains why this skill has been seen on troops of level 7 and higher but the heavy tank itself has only been seen in combat when used by non-player armies (just like the 
Fighter Jet).
Trivia
- It's not the only early XX century materiel seen in the game, which could probably mean than one of initial concepts was placed in a corresponding era hence some of the stylistic elements, but then revoked in favour of Modern one, giving a strange feeling visual mix.
Other Media
(Thanks to Chitzunr1 for the image.)