Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 454)

7 Custom Controls
454
During Movie Servo AF, you can pause the AF when you press the
button assigned to this function. Press the button again to resume
Movie Servo AF.
When you press the shutter button halfway, exposure metering is
performed (AF is not performed).
When you press the button assigned to this function, you can lock the
exposure (AE lock) while the metering timer is active. Useful when you
want to focus and meter the shot separately.
The exposure will be locked (AE lock) while you hold down the shutter
button.
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. Useful when you want to focus and meter the shot
separately or when you want to take multiple shots at the same
exposure setting.
When you press the button assigned to this function, you can lock the
exposure (AE lock) and the AF will stop. Useful during AI Servo AF if
you want to perform AE lock at the same time when AF stops.
: Pause Movie Servo AF
: Metering start
: AE lock
: AE lock (while button pressed)
: AE lock (hold)
: AE lock, AF stop
If you assign [ AE lock (while button pressed) ] to the shutter button, any
buttons assigned to [ AE lock ] or [ AE lock (hold) ] will also work as [ AE lock
(while button pressed) ].