Canon EOS Rebel SL3 DSLR Camera with 18-55mm Lens (Black) - Manual (Page 451)

451
C.Fn-7 Shutter/AE lock button
0: AF/AE lock
1: AE lock/AF
This is convenient when you want to focus and meter separately. Press
the <
A
> button to autofocus, and press the shutter button halfway to
apply AE lock.
2: AF/AF lock, no AE lock
During AI Servo AF (or Servo AF for Live View shooting), you can press
the <
A
> button to pause the AF operation momentarily. This prevents
the AF from being thrown off by any obstacle passing between the
camera and subject. The exposure is set at the moment the picture is
taken.
3: AE/AF, no AE lock
This is useful for subjects that keep moving and stopping repeatedly.
During AI Servo AF (or Servo AF for Live View shooting), you can
press the < A > button to start or stop the AI Servo AF operation. The
exposure is set at the moment the picture is taken. Thus, you can set
your camera to always maintain the optimum focusing and exposure
and wait for the decisive moment.
During movie shooting
z With setting 1 or 3, press the < A > button for One-Shot AF.