Canon EOS-1DX Af Setting Manualbook page 54

Single-point Spot AF
[Single-point spot AF] mode can be used
to focus on a small area of the subject.
Focusing on a small or narrow area
When set to [Single-point spot AF],
a small rectangle is displayed inside
the manually selected AF point.
[Single-point spot AF] mode makes it possible to focus
on the exact area even when there is an obstacle near
the area you want to focus on. A common example
when shooting sports is when you want to focus on the
eyes of a rider wearing a helmet. With normal settings,
the AF point can easily get caught on the edge of the
helmet near the eye, resulting in the camera focusing on
this edge. In situations like this, [Single-point Spot AF]
makes it possible to focus more accurately on the rider's
eye. As [Singlepoint Spot AF] only focuses on one very
small area, it is not really suited to capturing fast moving
subjects when set to AI Servo AF and may take longer to
focus than other AF area selection modes.
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