Canon EOS 90D Instruction Manual page 778

Custom Function Setting Items
C.Fn II-11: Initial Servo AF point,
You can set the initial AF point for AI Servo AF or Servo AF when the AF
area selection mode is set to [Auto selection AF], or when the AF method
is set to [ u +Tracking].
0: Auto

The AF point that AI Servo AF or Servo AF starts with is set automatically
to suit the shooting conditions.
1: Initial Servo AF pt for

AI Servo AF or Servo AF starts from the manually set AF point when AF
operation is set to [AI Servo AF] and the AF area selection mode to [Auto
selection AF], or when AF operation is set to [Servo AF] and the AF
method to [ u +Tracking].
2: AF pt set for

If you switch from [Spot AF] or [1-point AF] to [Auto selection AF] or
[ u +Tracking], AI Servo AF or Servo AF starts from the AF point that was
manually set before switching. Useful for starting AI Servo AF or Servo
AF from the AF point set before switching to [Auto selection AF] or
[ u +Tracking].
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