Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 395)

5.
Check the result of the adjustment.
Take a picture and view it ( ) to check the results of adjustment.
Adjust toward “+: ” to correct focusing in front of your targeted
position, and adjust toward “–: ” to correct focusing behind it.
Repeat the adjustment as necessary.
Caution
If [ All by same amount ] is selected, separate AF adjustment will not be possible
for the wide-angle and telephoto ends of zoom lenses.
395