Canon EOS-1DX MARK II Settings Manual page 111

Set up steps
1
There are two options to customize
the controls to register an AF point
A Assign
button, or the
select [Registered AF point]
B Assign
to the
info to select if the option is applied only
when the button is held or not
2
Manually select AF points you will
want to recall.
(This is possible with all AF area selection modes except Zone AF)
3
Press the
the
When the
the function [Metering - AF start], instant switching of
registered AF points becomes possible. Press the
button in the [Customize Controls] assignment screen,
and then select [Registered AF point]. Now when the
button is held the AF will use the registered AF point.
[Metering - AF start] to the
button then press info and
[Switch to registered AF point]
button,
button while pressing
button until you hear a beep.
button, or the
111
, or button. Press
button are assigned to