Canon EOS-1DX MARK II Settings Manual page 124

This feature makes it possible to
instantly switch between continuous
shooting modes using the two adjacent
buttons
Assign Assign
[Case1] to [Case2] to
When usually using the button (Case1) to
shoot, use the button (Case2) to shoot
when there are many obstacles
Example:
Switch AI Servo AF characteristics with
the button and button
When you assign functions to the and buttons,
those two adjacent buttons can be used to instantly
switch functions for shooting. For example, when
assigning the two often used AI Servo AF characteristics
(Cases) to each button, it is possible to switch to the
optimal AI Servo AF characteristics depending on the
subject and if any obstructions may enter the frame.
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