Canon EOS REBEL T8i Advanced User's Manual page 170

Auto selection AF
The Area AF frame (entire AF area) is used to focus.
With [One-Shot AF], generally focuses on the nearest subject. Note that any detected
faces are given priority in focusing.
With [AI Servo AF], focusing continues as long as the Area AF frame can track the
subject.
Caution
AF points may not track subjects under some shooting conditions when [AI Servo
AF] is used with [Manual select.:Zone AF], [Manual select.:Large Zone AF], or
[Auto selection AF].
Focusing may be difficult when using a peripheral AF point or a wide-angle or
telephoto lens. In such a case, use the center AF point or an AF point close to the
center.
When the AF point(s) light up, part or all of the viewfinder may light up in red, which
is a characteristic of AF point display.
In low temperatures, the AF point display may be difficult to see or have a slower
tracking response due to the nature of the AF point display device (using liquid
crystal).
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