From Final Fantasy Wiki
Automatons are the mechanical pets of Puppetmasters in Final Fantasy XI. They can be summoned into battle once every 20 minutes. They can only be called if the zone is one where combat can occur, that is, only outside of cities with the exception of city zones that can be sieged.
Automatons can have 3 distinctly different chassis. Each chassis has a different statistical set. The Valoredge X-900 excels at melee combat but due to low defense is not able to tank with any effectiveness. The Sharpshot Z-500 chassis excels at ranged combat and is the most powerful (damagewise) frame with high level weaponskills such as Armor Piercer. Finally, the Stormwaker Y-700 is a magic using chassis. The Stormwalker Y-700 can attach a Spirit-Reaver or a Soul-Soother head piece to specialize in Black or White magic spells.
The chassis themselves are kind of like jobs for the puppet. Stats and abilities vary between the three chassis. Also, the type of chassis determines what type of attachments can be installed in the puppet.
When finishing the quest to become a puppetmaster, the player gets an option as to what they would like to name their automaton. Below are the lists. Initially only List 1 is available, but a player can then earn access to the other lists, 2 through 4.