.sfc is the extension for SFC files, the world save format for Creatures 1 and Creatures 2. They contain all modifiable information about worlds - such as rooms, agents and creatures. The default world - which the Emergency Kit and World Switcher use to reset or make a new world - is stored in Eden.sfc, and the current world in World.sfc - although you can also use different files if you're using cheats or the World Switcher. Third-party worlds come with their own sfc files.