The FLAT (Hangul: 플랫, Peul-laet) is described as a virtual space for the æ. Using SYNK, the place is used for the people and their æ to communicate with one another. A person from the REAL WORLD can only go to FLAT once they SYNK DIVE into their æ, but their physical body will stay behind. The people and æ, SYNKed with one another, can also use their virtual powers in FLAT.
In Black Mamba, the lyrics "You’re in the FLAT" are referencing to this location.
Supposedly, the FLAT was first shown in the Black Mamba music video during the first minute of the video as a gradient background if blue and teal with pink spots. It was then fully shown in episode 2 of the SM Culture Universe series.