In Elysium, for those born in the Lower Sectors, their existence is reduced to a number. They are given a Citizen Number, an impersonal string of digits that replaces their identity, stripping them of any sense of individuality. In contrast, those born in the Upper Sectors are granted the privilege of a name, a true identity, signifying their higher status.
This system serves to reinforce the rigid class structure, where the Lower Sectors are seen as mere statistics, while the Upper Sectors are treated as individuals. For people like Isaac, the Citizen Number is a constant reminder of his subjugation. As he mentions in the first chapter: "Isaac. She always uses my chosen name, never my citizen number – #24-601. Another small act of rebellion." Isaac is the name Mia gave him, as a way to show him that he is the same as her, despite their different social standings. Although this act isn't explicitly shown in the novel, it represents a subtle yet powerful act of defiance against the oppressive Citizen Number system.