Canon EOS REBEL T7I Basic Instruction Manual page 102

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>.
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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.98) AF
point <S> is used first to achieve focus. The AF
point(s) achieving focus is displayed as <S>.