Canon EOS REBEL T8i Advanced User's Manual page 182

3.
Focus and take the picture.
Aim the AF point over the subject and press the shutter button halfway.
When focus is achieved, the AF point will turn green and the beeper
will sound. If focus is not achieved, the AF point will turn orange.
Caution
AF points may not track subjects under some shooting conditions when [Servo AF]
is used with Zone AF.
Focusing may be difficult when using a peripheral AF point. In this case, select an
AF point in the center.
182