The Far Side is an Imaginary Space

The setting of CCC. If one thinks of the moon as having two sides, the side facing Earth is the Near Side, while the opposite...the side that cannot be observed from the Earth is the Far Side. While Fate/EXTRA starts from the first floor of the Near Side and heads for the seventh floor ~ the core, CCC starts from the Far Side (no concept of floors). Though the path may differ, the end goal for both is the core of the Moon Cell. The Far Side of the Moon is a place where data deemed unnecessary by the Moon Cell is kept, similar to a dumping ground. It is imaginary number space that continuously takes in data categorized as malignant information――that which is nothing but harmful to humans as well as intelligent lifeforms in general. The Far Side is territory so taboo that even the “Eye” in the core of the Moon Cell is forbidden to enter. Data that have entered this place, after becoming engulfed in humanity’s malignance and losing sight of their purpose of existence, ultimately become deprived of meaning. The Far Side can also be said to be an ocean that has amassed the pollutants that the Moon Cell accrues as it makes its observations.

-Fate/EXTRA material - Encyclopedia: Far Side of the Moon [Circumstances], p.198-199