Canon EOS-1D X Mark III Advanced User's Manual page 328

Note
Using these selection methods, you can manually select the initial AI Servo AF
position when [
/ ] ( ).
Pressing the <
• Zone AF, Large Zone AF, and Automatic selection AF:
• Spot AF, 1-point AF, AF point expansion:
center)
: Initial Servo AF pt,
> button displays following information on the top LCD panel.
/ ] is set to [Initial AF pt set for
(Center)/
328
(Off