Canon EOS-1 DX Mark III Advanced User's Manual page 332

Spot AF (Manual Selection)
The camera focuses in a narrower area than 1-point AF. Select one AF point < > for
focusing.
Effective for pinpoint focusing or focusing on overlapping subjects such as an animal in a
cage.
Note that the small Spot AF area may make focusing difficult in handheld shooting or for
moving subjects.
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