C.Fn-7:If an external power source is used, the flash recycling is powered concurrently by the internal batteries and external power source. In this case, when the internal batteries become exhausted, shooting might not be possible. If 1 is set, the flash recycling will be powered only by the external power source. The internal batteries will thereby last longer. Note that even when 1 is set, the internal batteries must still be installed to power the flash control.
C.Fn-9:If 1 is set, you can press the shutter button halfway and press it twice to turn on/off the focusing lamp. This is convenient when both hands are occupied.
C.Fn-3-1 is mainly for EOS-1 series film cameras. Do not use C.Fn-3-1 with EOS DIGITAL cameras or with the EOS REBEL T2/300X. If C.Fn-3- 1 is set for such cameras, the flash control will not work properly. The flash might not fire or it might fire only at full output.
With Type-A cameras, if C.Fn-3-1 is set, wireless autoflash shooting will not be possible.
If C.Fn-5-1 is set, single flash tube/head firing will not be possible. If C.Fn-9-1 is set, take care when pressing the shutter button. For example, if you keep pressing the shutter button halfway at short intervals to autofocus, you might inadvertently turn on/off the focusing lamp. This can also occur with the AE lock button and depth-of-field preview button if you press twice one of them or press one button after another at short intervals.
With Type-B cameras, even if C.Fn-3-0 is set, E-TTL II/E-TTL autoflash will not work.