Canon EOS-1DX MARK II Settings Manual page 60

AF Point Expansion provides a larger cluster of active AF
points — either four or eight surrounding, "assist"
points — in addition to your central, primary AF point.
AF always begins with the central primary AF point
alone. If this AF point is either moved away from the
subject momentarily, or falls upon a plain area of a
subject with little detail or contrast, the AF system
immediately calls upon the sourrounding four or eight
AF points to try to continue to focus upon the subject.
AF Point Expansion can be especially effective with
moving subjects, particularly small subjects that may be
hard to keep a single AF point upon (like birds in flight,
for example). Likewise, it can be useful for sports and
similar subjects, when there's not a lot of obvious detail
to focus upon.
Based on the subject's movement characteristics, (i.e.
likelihood of subject moving from the selected AF point)
and the size of the subject within the frame, select either
the 4-point or 8-point AF Expansion option.
In addition, when the subject has a lot of side-to-side
movement, setting Case 5 or Case 6 from the AF
Configuration Tool is also recommended.
60