Canon EOS-1D X Mark III Advanced User's Manual page 205

Flash mode
You can select the flash mode to suit your desired flash photography.
[E-TTL II flash metering] is the standard mode of EL/EX series Speedlites for automatic
flash photography.
[Manual flash] is for setting the Speedlite's [Flash output level] yourself.
[CSP] (Continuous shooting priority mode) is available when using a compatible external
Speedlite. This mode automatically reduces flash output by one stop and increases ISO
speed by one stop. Useful in continuous shooting, and helps conserve flash battery
power.
Regarding other flash modes, refer to the Instruction Manual of a Speedlite compatible
with the respective flash mode.
Caution
Adjust exposure compensation (
photography with [CSP] in <
Note
With [CSP], ISO speed is automatically set to [Auto].
) as needed in case of overexposure from flash
> or < > mode.
205