Decades ago, the earth's resources suddenly depleted, and humans had no choice but to use the Ark, which was still under construction. The Starfleet Fleet escorted the surviving species in batches. When it arrived, the Ark could only open three areas that could be used by the public, and the remaining seven were still in the initial construction stage. Therefore, the Starfleet Fleet collectively referred to the boundaries of the three areas as the "ground line." Outside the ground line, it was full of dangers.
Purpose: Change is permanent and we have to accept it