Canon EOS-1 D X Mark II Instruction Manual page 95

AF Point Display Indications
Pressing the <S> button lights up the AF points that are cross-type AF
points for high-precision autofocusing. The blinking AF points are
horizontal-line or vertical-line sensitive. For details, see pages 100-108.
Registering the AF point
You can register a frequently-used AF point to the camera.
When you use the button set with the [86: Custom Controls] (p.443)
menu's detailed settings screens for [Metering and AF start], [Switch
to registered AF point], [Direct AF point selection], or [Register/
recall shooting func], you can instantly switch from the current AF
point to the registered AF point.
For details on registering the AF point, see page 448.
S Selecting the AF Area and AF Point
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