From Halopedia, the Halo wiki
The holotank, also referred to as a holo-projector, is a device used to project a hologram. Small holotanks commonly used by AIs are often referred to as holo-pedestals, while table-sized ones are known as holotables.
Most UNSC ships and facilities are equipped with holotanks to display information such as tactical readouts, holographic communiques, or mission recordings. UNSC holotanks vary greatly in size and shape, ranging from small cylindrical pedestals reserved for AI avatars to large tables used to display tactical information on the bridge of a warship, for example.
The Covenant commonly use holotanks on their starships, and they are plentiful in High Charity as well. AIs can also access the map data from the holotank's link to the Covenant information network and give directions verbally to anyone near the holotank. During the Fall of High Charity, Cortana entered one to aid John-117's passage through the city, remaining there long after the space station was transformed into a Flood hive. The Covenant also utilize portable holo-pedestals to communicate over distance or in the battlefield. The Luminaries mounted on Covenant ships are typically linked to the bridge's holotank, which can display the information detected by the Luminary.
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