From Halopedia, the Halo wiki
The Type-27 Responsive Holographic Form Emulator (T-27 RHFE), often shortened as "Hologram" or "T-27 Decoy package", is a piece of Covenant decoy technology. It was reverse-engineered by the UNSC, and is utilized by SPARTANs.
The only known Covenant species that uses the emulator are the Skirmishers, namely only the Skirmisher Champions and Commandos. Sometime before or during 2552, the emulator was reverse engineered from Covenant technology and was later integrated into the MJOLNIR Powered Assault Armor. The emulator creates an holographic copy of the user and can be commanded to move to locations chosen by the user. It is often used to deceive enemies.
To allies, the holographic copy would appear as a white dot on their motion tracker. The ability functions by pointing the targeting reticule at a destination and deploying it like other armor abilities. When activated, the hologram will run in a straight line to the destination and stop once it reaches its designated location. The hologram will flicker away if it is either damaged enough or if it reaches its lifespan limit (10 seconds). However, a new hologram can be produced after just 7 seconds, which would immediately dissipate the old one.
The hologram can be used to bait enemies into shooting it, while you peek around cover and shoot them for being foolish enough to shoot at it.
The decoy can be identified by the thermal enhancement on the SRS99AM Sniper rifle's Oracle N-variant scope. On the scope, decoys appear as flickering white solids. The decoy will also "run" nonstop, no matter what might be in its way (including walls or other terrain), and will only run in a straight line at a measured pace, allowing it to be distinguished from a real enemy after a few seconds of observation.
List of appearances