Flash and Focus assist light set to auto: Flash and focus assist light are both automatically on when needed. If it’s dark, the focus assist light lights up first while the camera is trying to focus, after which the flash fires when you take a photo.
Flash light is On and Focus assist light is in Set by flash (or Use Flash setting): The flash always fires, even during daylight. The focus assist light may or may not light up. Keeping the flash on is useful in backlight situations when the subject is dark and a bright light is coming from behind.
Focus assist light only: Flash is off, but the camera uses focus light when needed (for example, when it’s dark). This is useful when you want to use natural light and focus quickly.
No Focus assist light: Focus assist light is switched off, but the flash is on Auto. This is useful if you don’t want to annoy people with the focus assist light but still want to use the flash to light your subject.
Off: Both flash and Focus assist light are off. This is useful in situations where using a flash is forbidden, for example, in art galleries.
Note: When the focus assist light is set to Set by flash (or Use flash setting), the focus assist light is only allowed if the flash setting is enabled.