As we saw in the previous preview from WizKids about their upcoming Convention Exclusives for HeroClix, Pym Particles can be used to make things smaller, or they can be used to make things much, much bigger. That doesn't just include Hank Pym, himself, but various objects as well, including tanks.
The Pocket Tank isn't much to look at. Basically, it's just there to (hopefully) be turned into the Pym Particle Tank, thanks to its "... than can return to its original size" ability. That's when the "actual" tank comes out. Being a vehicle, the Pym Particle Tank has a couple different ways it can be run, depending on if you want to have it piloted or acting on its own. Obviously, actually having someone behind the wheel (or steering sticks, as it probably has) grants you better abilities, but at the cost of having someone tied to the tank.