From Halopedia, the Halo wiki
They are found within High Charity and in various other Covenant ships. They are used for holding traitors, prisoners of war and convicts. They have an energy barrier on the front, which can only be deactivated with a switch, in the case of the Ket-pattern battlecruiser. Shooting these barriers will cause them to flicker a dark blood red and make all ammunition being fired at it, besides plasma, ricochet off. However, on High Charity the prison cells had bars on the front, much like modern day jail cells.
The Covenant also use a type of field projector to create temporary holding cells within existing structures in the field. Suspended in mid-air, the device generates an energy barrier which can be used to wall off prisoners and can only be deactivated from the other side. The Covenant used this type of temporary holding cell during the a skirmish in Tanzania on November 17, 2552, when they captured Sergeant Johnson and his men in a small room within A2 Hydroelectric Facility.
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