Canon EOS-1DX MARK II Settings Manual page 113

Another function that is effective for quickly switching
AF points while shooting is AF point registration and
recall feature possible using [Custom Controls]. There
are several methods to achieve this; one is to assign
registered AF points to a button via [Custom Controls]
function. The second method is AF point or [AF area
selection mode] registration. By carrying out either of
the two options, you can press a button and instantly
switch to registered AF points.
Registering frequently used AF points, or a strategically
placed AF point, enables instant response without the
need to reframe or alter the cameras position. Further
refinement in operation is possible, with the <Depth-of-
field preview>, and the < > buttons when set to
([Switch to registered AF point] setting). These buttons
provide the possibility to [Switch only while pressed] or
[Maintain switching until pressed again] settings,
making detailed customization possible. Using these
settings enables the camera suit the way you shoot.
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