Canon EOS-1D X Mark III Advanced User's Manual page 330

Registering AF Points
You can register the position of a frequently used AF point. For instant switching from the
current AF point to your registered AF point, you can use a button you assign on advanced
setting screens for [Metering and AF start], [Switch to registered AF point], [Direct AF
point selection], or [Register/recall shooting func], in [ 6: Custom Controls] ( ).
For instructions on registering AF points, see Custom Controls.
330