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Gndlf
It depends if you have CYO T or just a CYO.
For the CYO there's O, 1 and S (instructions here).
O - off.
1 - on all the time.
S - senso. It will turn on when it thinks it's dark enough and off in the light
For the CYO T, you have a different set of three - O, S and T (instructions here). This light has two modes, night lights and daylight running lights (taglicht, DRL). In night mode the extra DRL LEDs are dim and the main beam is on full. In DRL mode, the main beam is dimmed and the extra LEDs run full blast. You can read more about the DRL mode here.
O - off.
S - senso. It switches automatically between the daylight running light mode and full main beam according to a light sensor.
T - Taglicht. This keeps the headlight in DRL mode, even at night.