Tau Drones
Drones are small, round, usually hovering robots used by the Tau Empire for various support tasks. Because the Tau have never had to experience a robot rebellion (yet) the way that the Imperium of Man did with their Men of Iron, artificial intelligences are not only still permitted but widespread in society. Unlike servitors, drones are capable of autonomous activity. They aren't very bright when they work on their own, but they can interconnect into networks and become more intelligent. Despite being common and mass-producible, drones are not regarded as expendable and are programmed for self preservation. The average player doesn't share this sentiment, and when it comes to saving valuable personnel's lives (like battlesuit pilots or squad leaders), neither do Earth caste engineers as they often programm drones to jump at enemy bullets and missiles to save said lives, and some of the drone types are even specifically designed to be used as meatshields nano-crystaline alloy armored shields.
Command-link Drone
Commander Shadowsun uses a special drone with extra communication equipment to help direct her soldiers. In-game, it used to project an 18" Ld10 bubble, but now it lets a unit re-roll To Hit rolls of 1 by virtue of giving the squad her targeting data and the like (because having Shadowsun shouting in your ear prevents that Heavy Burst Cannon from getting hot, or something).
Drone Sentry Turret
Drone sentry turrets are probably the largest autonomous systems used by the Tau Empire, used to secure perimeters without requiring Fire Warriors to be put in harm's way - this is because the Tau shun the concept of static fights, so they let these drones do it. As for why there are new Tau fortifications, that's because GW doesn't seem to communicate with FW, and even then, those fortifications aren't immobile. Back to the topic, these drones mount twin-linked burst cannons, missile pods, fusion blasters, or plasma rifles in a "pop-up" configuration, so that the entire turret stays as enclosed as possible until the moment the guns need to fire. Being considered vehicles, they also have access to vehicle systems.
Exploratory Drone
DX-11 Exploratory Drones are basically Tau version of C.A.T. units in Space Hulk missions, and fit similar roles of scouting, exportation and locating targets valuable enough to send a Fire Warrior team.
Grav-inhibitor Drone
Grav-inhibitor drones incorporate a "gravity wave projector" that slows down assaulting units, allowing the cowardly and weak intelligent Tau to blast the assaulting unit to kingdom come via overwatch survive just a little longer in the face of the Emperor's close-combat might.
Let's be real they probably use it mostly on orks.
Gun Drone
Gun drones are the most common sort of drone encountered on the battlefield, armed with a twin-linked pulse carbine. They can be attached to squads of Fire Warriors, or fixed onto vehicles as detachable turrets, or even operate independently in squads of their own. Most other drones are built on the same chassis as the gun drone.
Heavy Gun Drone
The heavy gun drone is, as it sounds, the big brother of the gun drone, and in fact is the largest model of mobile drone used by the Tau. They carry twin-linked burst cannons, and can swap one of them for a markerlight if it will need to call down the seeker missiles.
Marker Drone
Marker drones are modified gun drones that carry markerlights for their squad (why the lazy bums couldn't carry their own markerlights is anyone's guess). They're more expensive than Pathfinders, but they can tag along with whoever is controlling them (which means, if they run with battlesuits, they can jump-shoot-jump as drones are basically independent jet packs).
Missile Drone
Missile drones carry missile pods. Shocking.
What's really shocking is that the Tau only deploy them with Broadside battlesuits (well, Games Workshop ruled that they do). Apparently, swarms of S7 AP4 was too awesome for a xenos army.
Pulse Accelerator Drone
Pulse accelerator drones are drones that carry a pulse accelerator, which accelerates pulse rounds. Redundancy aside, it extends the range of pulse weapons. Only Tau Pathfinders use them.
Recon Drone
Yet another toy for Pathfinders, the Recon Drone takes the bold step of giving squads organic heavy weapons -- well, a burst cannon isn't any more powerful than a pulse carbine, but it puts more shots downrange. It comes with some other toys for bringing in reinforcements, like a homing beacon and a positional relay (read: bubble of no-scatter for Deep Strikers).
Remora Drone Fighter
The DX-6 Remora Drone Fighter is a drone flyer, the main body about the same size as the Heavy Gun Drone, but it has wings, larger engines to enable true flight (so it's the biggest drone), and more weapons, being able to carry a twin-linked long-barreled (36" Heavy 6) Burst cannons, a networked Markerlight, and two Seeker missiles at the same time. They're packed full of stealth technology so it can provide close air support Stealthsuit and Pathfinder teams.
They are very lightly armored, but they don't need to take enemy fire on the way to battle. Instead, they are often carried in large numbers by a Manta or in pairs by a Tiger Shark AX-2-2 (their wings can fold up and over the body like IRL naval aircraft), which will release them from high above the battlefield and control them from afar.
Real-life Remoras are a kind of fish that hangs on to larger fish for transport and protection. Their secret life as artificially intelligent stealth fighters is as of yet unconfirmed, probably because they're stealthy as well.
Remote Sensor Tower
Remote sensor towers take the prize for the tallest drones used by the Tau -- when collapsed, they are twice as tall as a Fire Warrior, and they are twice as tall again when fully extended. They carry twin-linked markerlights and forward targeting data to nearby formations. They are often placed with sentry turrets to act as an autonomous defense line; if heavy anti-vehicle firepower is required, they can summon seeker missile-bearing Piranhas.
Shield Drone
Shield drones carry shield generators for protecting high-value targets (read: 4++ invulnerable save to soak high-Strength and low-AP attacks). The latest version is the MV52, which incorporates a 3++ save -- but only for Commander Shadowsun (which get kind of ridiculous in a unit that already has stealth and shrouded).
Shielded Missile Drone
The shielded missile drone was born in a moment of brilliance, when Earth Caste engineers realized that they could stick two systems on a single drone. The brilliance was immediately canceled out when the Ethereals decreed that the new drones would only be used by Riptides and their variants.
Sniper Drone
Sniper drones carry a rail rifle longshot pulse rifle (arguebly one of the best sniper weapons in the game, being AP5 instead of 6 48" range and RAPID FIRE! Yeah, two sniper shots at BS5 in 24" range from one drone) and come in teams led by up to three spotters carrying a markerlight/drone controller device that looks like a Virtual Boy.
Technical Drone
DX-4 Technical drones are the only drones without rules of their own -- Forge World suggests using them to represent certain stratagems in Cities of Death games, and there's an Apocalypse formation that uses them to repair drone sentry turret, but for regular games of Warhammer 40,000, they're really only good for decoration. Maybe they could be used by an Earth Caste who counts as a tech-priest or Techmarine from an allied detachment, or just homebrew a similar Tau unit.
Gallery
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Shadowsun's personal Shield Drone.
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Drone Sentry Turret: How Tau hold ground.
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A Grav-Inhibitor Drone - Perfect for ruining a good charge.
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The infamous Marker Drone, essential component of cheesy tau commander builds everwhere.
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Pulse Accelerator drone - A great piece of boosting tech, tho hampered by only being able to go with Pathfinders.
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Sniper Drone Squad with controller. What was it with synthetic and organic life not being able to co-operate?
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Shielded missile drones, the creme de la creme of Drones.
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Technical Drones: The wet dream of any Tau converter.