Cryo B was a cryogenic stasis chamber on board the Halcyon-class light cruiser UNSC Pillar of Autumn. John-117 was placed here during the trip from Reach to Halo. Following standard UNSC procedure, Keyes jettisoned most of the Cryo-chambers upon arrival in-system  and there were only nine of them left when they found Installation 04.
Cryo B has two sections, separated by several corridors at the bottom, and the Main terminal at the top. Each section holds 28 cryo-tubes.
The first side is where John awoke and tested his armor shields, whereas is likely the other side was used for the "weapons diagnostics" mentioned in the beginning of Pillar of Autumn.