Canon 600EXII-RT Instruction Manual page 116

C: Setting Custom Functions
C.Fn-12:
0: 1 (External & internal power)
Charges using both internal and external power sources.
1: 2 (External power only)
Internal power source is needed to control the Speedlite. By using
external power source for charging firing of the Speedlite, you can
minimize the depletion of the internal power source.
C.Fn-13:
0: 4 (Speedlite button and dial)
1: 5 (Speedlite dial only)
You can directly set the flash exposure compensation amount and
flash output by turning <9> without pressing the <@> button.
C.Fn-20:
You can enable a beep to sound when flash recharge is complete.
0: OFF (Disable)
The beep does not sound.
1: ON (Enable)
In normal shooting (on-camera flash photography), the beep will sound
when the flash unit is fully charged.
When the Speedlite is set as the master during radio transmission
wireless shooting, the beep will sound on the master unit when all flash
units (masters and slave units) are fully charged. You can confirm
recharging of the entire wireless system by the beep tone on the
master. For the slave units, either 0 or 1 can be set for C.Fn-20.
When the Speedlite is set as the master during optical transmission
wireless shooting, a slave in radio transmission/optical transmission
wireless shooting, or a master/slave in linked shooting, a beep will
sound on each flash set to 1 at full charge (p.84).
When 1 is set, the beep will also sound when the temperature of the flash
head rises and flash firing is restricted (p.124).
116
3
(Flash recycle with external power)
6
(Flash exposure compensation setting)
7
(Beep)