Heavy AATs featured superior firepower and durability than the AAT. It was extremely effective against heavy Republic vehicles such as the AT-TE walker and Republic artillery.
OOM-9 used a Heavy Armored Assault Tank to capture several Naboo Cities like Parrlay, New Centrif, Vis and Harte Secur during the Invasion of Naboo.
The Heavy AAT also saw action during the Liberation of Spinnaker and Liberation of Harte Secur, (along with AATs), where these tanks were very effective against the Gungan units, destroying many of them, but, also many Heavy AATs were destroyed by the Gungan Grand Army.
In Star Wars: Galactic Battlegrounds, OOM-9 uses a Heavy AAT during early levels, but later uses a standard AAT during the Battle of Naboo.