Canon EOS 6D Mark II Mode D'emploi page 110

f: Selecting the AF OperationN
AI Servo AF for Moving Subjects
The exposure is set at the moment the picture is taken.
When the AF area selection mode (p.113) is set to Automatic
selection AF, focus tracking will continue as long as the Area AF
frame covers the subject.
AI Focus AF for Switching the AF Operation Automatically
After the subject is focused in One-Shot AF, if the subject starts
moving, the camera will detect the movement, change the AF
operation automatically to AI Servo AF, and start tracking the moving
subject.
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This AF operation is suited for
moving subjects when the focusing
distance keeps changing. While you
hold down the shutter button halfway,
the camera will keep focusing on the
subject continuously.
AI Focus AF switches the AF
operation from One-Shot AF to AI
Servo AF automatically if a still
subject starts moving.