Canon EOS 80D - Manual (Page 114)

f : Selecting the AF OperationN
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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.
 The exposure is set at the moment the picture is taken.
 When the AF area selection mode (p.114) is set to 45-point
automatic selection AF, focus tracking will continue as long as the
Area AF frame covers the subject.
AI Focus AF switches the AF operation from One-Shot AF to AI
Servo AF automatically if a still subject starts moving.
 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.
AI Servo AF for Moving Subjects
AI Focus AF for Switching the AF Operation Automatically
With AI Servo AF, the beeper will not sound even when focus is achieved.
Also, the focus indicator <o> in the viewfinder will not light up.
When focus is achieved with AI Focus AF with the Servo operation active,
the beeper will continue beeping softly. However, the focus indicator <o> in
the viewfinder will not light up. Note that focus will not be locked in this case.