C.Fn-02-3: Convenient when you want to check the depth of field. (p.44)
C.Fn-12: 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 first, 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 if you set it to 1, the Speedlite will still require internal batteries for flash control.
C.Fn-05-1 is geared for EOS-series film cameras.
Do not set it if you have a, EOS DIGITAL camera or the EOS REBEL T2/ 300X. If C.Fn-05-1 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-05-1 is set, wireless autoflash shooting will not be possible.
If “AF-assist beam OFF” is set with the Speedlite or camera, the AF- assist beam will not be emitted.
With Type-B cameras, even if C.Fn-05-0 is set, E-TTL II/E-TTL autoflash will not work.