Canon 5D Mark III User Manual (en)

82: Custom Controls
During flash photography, pressing the button assigned to this function 
will fire a preflash and record the required flash output (FE lock).
You can set the ISO speed by holding down <
0> and turning the 
<6> dial. If Auto ISO is set, manual ISO speed setting will take effect. 
Auto ISO cannot be set. If you use this function in the <a> mode, you 
can adjust the exposure with the ISO speed while maintaining the 
current shutter speed and aperture.
During metering, you can set the ISO speed by turning the <5> dial.
If Auto ISO is set, manual ISO speed setting will take effect. Auto ISO 
cannot be set. If you use this function in the <a> mode, you can adjust 
the exposure with the ISO speed while maintaining the current shutter 
speed and aperture.
In manual exposure <a>, you can set the shutter speed with the 
<6> or <5> dial.
In manual exposure <a>, you can set the aperture with the <6> or 
<5> dial.
: FE lock
: Set ISO speed (hold button, turn 
: Set ISO speed (  during metering)
: Shutter speed setting in M mode
: Aperture setting in M mode