The Phantom Zone is a "pocket universe" used as a humane method of imprisonment due to Krypton abolishing the death penalty years prior.

[2] The inmates of the Phantom Zone reside in a ghost-like state of existence from which they can observe, but cannot interact with, the regular universe.

[13] In the "New Krypton" storyline, the Kryptonians in Kandor capture Superman's enemies to throw them into the Phantom Zone, including Parasite, Silver Banshee, and Toyman.

[19][20] In DC Rebirth, Superman theorizes that the Phantom Zone is a permeable membrane between the main universe and the Dark Multiverse.

The judges reason that even if Superman feels that he may have killed his foe deliberately after the deaths of his family and friends, putting him in a conventional prison without his powers would be dangerous and solitary confinement was too extreme given his past deeds, selecting the Zone based on the suggestion of the new Batman, Bruce Wayne Junior.