Those who scored within the middle percentile are given the job of laborers. These people raise the cities' artificial food source (which was developed by the government) as well as manufacture their torches,
The ones who score in the lowest percentile essentially become servants to the high-ranking officials of government. Likewise, they are also tasked with constantly replacing the burnt out torches with new ones.
Despite the loss of the sun, people still managed to adapt to their living conditions and develop technologies that aided in their everyday needs. The people of this society were able to reinvent tracks and carts that help them travel from one city to another. Likewise, the government was able to create a small miniature "sun" that can emit artificial sunlight.
While friendships/relationships are not strictly forbidden, the overarching philosophy of individualism prompts children to berate those who match their intelligence (as they become a threat to their potential spot in government). After their jobs are assigned, they branch out more and begin to develop more relationships.
Those who decide to rebel or refuse to work for the cause (building an artificial sun) are banished from society and are sent back up to the surface to die from harsh conditions.
The main goal of the society is to be able to construct an artificial sun large enough to revert earth back to its former state. So far, they have been able to construct a small artificial one that was made through electrical components collected through the years.