Canon EOS REBEL T7i Instruction Manual page 128

AF Area Selection Modes
o Automatic selection AF
The Area AF frame (entire AF area) is used to focus. The AF point(s)
achieving focus is displayed as <S>.
When AI Servo AF mode is set for Zone AF, Large Zone AF, or Automatic

selection AF, the active AF point <S> will keep switching to track the
subject. However, under certain shooting conditions (such as when the
subject is small), it may not be able to track the subject.
If a peripheral AF point or a wide-angle or telephoto lens is used,

achieving focus may be difficult with an EOS-dedicated, external
Speedlite's AF-assist beam. 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. This is a characteristic of AF point display.
In low temperatures, the AF point's display may be difficult to see. This is

a characteristic of AF point display. Also, the tracking response may
become slower.
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With One-Shot AF, pressing the shutter button
halfway will display the AF point(s) <S> that
achieved focus. If multiple AF points are
displayed, it means they all have achieved focus.
This mode tends to focus on the nearest subject.
With AI Servo AF, the manually-selected (p.125)
AF point <S> is used first to achieve focus. The
AF point(s) achieving focus is displayed as <S>.