From Halopedia, the Halo wiki
They are used for several functions, allowing face-to-face communication over relative short distance, providing holographic duplicate decoys of important personnel, and engaging enemy forces without risking allied personnel.
Sesa 'Refumee, the Sangheili leader of a faction of heretics based on Basis and a gas mine on Threshold, used one of these to taunt a Covenant Special Operations assassination team, and later used another two to assist him in battle as he tried to kill the Arbiter but failed.
They are relatively weak in battle and a direct shot will result in static washing over their form, betraying their holographic nature. By using an unknown technology, these holo-drones can interact with the environment around them and attack and even physically harm their targets. Though they possess rudimentary shields, and are somehow capable of using actual weapons, they do not seem to be dedicated combat equipment. Physically, they resemble inactive plasma grenades able to fly through the air fairly quickly, though slightly larger.
- The holo-drone may have been inspired by similar holographic decoy technology featured in the 1990 film Total Recall.
- The Hologram armor ability appears to be a less sophisticated version of this technology, producing a holographic decoy of the user that is only able to move in a straight line.
- The holo-drone's ability to physically interact with its environment and even directly inflict damage suggests that this may be accomplished through the use of hard light.
- In the Halo 2 campaign level The Oracle, there is a glitch allowing the player to find the holo-drone near the beginning of the level without triggering the scripted sequence in which the drone is activated.
A decoy of Heretic Leader Sesa 'Refumee produced by a holo-drone