From Halopedia, the Halo wiki
UNSC recruiting centers are automated kiosks placed in human population centers, allowing citizens to enlist in the various branches of the United Nations Space Command.
There were several recruiting centers in New Mombasa, often with anti-UNSC graffiti sprayed next to them.
Initial concept art of the recruitment kiosks.
Concept art of a vandalized recruiting center.
An advertisement for the UNSC Navy in a recruiting center terminal.