Canon EOS 5D Mark III Instruction Manual (en)

82: Custom Controls
When you press the button assigned to this function, you can lock the 
exposure (AE lock). The AE lock will be maintained until you press the 
button again. Convenient when you want to focus and meter the shot at 
different areas or when you want to take multiple shots at the same 
exposure setting.
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.
: AE lock (hold)
: FE lock
: Set ISO speed (hold button, turn 
: Set ISO speed (  during metering)
: Shutter speed setting in M mode
: Aperture setting in M mode