Canon EOS-1DX MARK II Settings Manual page 58

Single-point AF
Single-point AF is an easy to use mode for still life
photos, etc. in One-shot AF
Single-point AF is a mode where one
manually selected AF point is used to
focus.
Using a full, single AF point is the EOS-1D X Mark II's
default AF Area setting. You're always using one AF
point at a time, and can manually move it to any of the
availble 61 AF point locations.
For experienced photographers, or when it is easy to
track the subject with a single AF point, AI Servo AF can
be utilized when continuously shooting moving subjects.
However, this mode is more effective for shooting still
life and landscapes with One-shot AF mode.
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