The format is a file standard that stores sample-based instrument data. Unlike a simple WAV file, an SF2 contains the raw audio samples along with parameters like loop points, filter envelopes, and velocity layers.
The factory presets were terrible. Thin pianos, anemic strings, a “Rock Drum” kit that sounded like cardboard boxes falling downstairs. Jun was about to turn it off when he noticed a tiny, scratched label near the data wheel: SF2 Custom Bank #17 – K. Yamaoka. korg sf2