Sensor Watch Firmware
While you can build any number of bare-metal applications for Sensor Watch, when we refer to the Sensor Watch firmware, we're really talking about Movement, the community firmware for Sensor Watch. You can read more about Movement here; for now, the important thing to know is that it manages a series of watch faces that you advance through using the mode button. It's a very similar idiom to the classic Casio F-91W, which advances from Clock -> Alarm -> Stopwatch -> Time Set, and then wraps around to Clock.
Movement offers many more watch faces — some of which, like World Clock, you can include more than once! Still: you'd be pressing the Mode button for hours if we included them all, so instead, we only build a subset of the available faces into any given firmware. This means that you can download an alternate firmware that includes the kind of functionality you want, or build custom firmware that includes just the watch faces you desire.