Canon EOS REBEL T8i Advanced User's Manual page 216

Slow Synchro
You can set the flash-sync speed for flash photography in <
[ ] 1/200-30sec. auto
The flash sync speed is set automatically within a range of 1/200 sec. to 30 sec. to suit
the scene's brightness. Slow-sync shooting is used in low-light locations (under some
shooting conditions), and shutter speed is automatically lowered.
[ ] 1/200-1/60sec. auto
Prevents a slow shutter speed from being set automatically in low-light conditions. It is
effective for preventing subject blur and camera shake. However, while the subject will
be properly exposed with the flash, the background may come out dark.
[ ] 1/200 sec. (fixed)
The shutter speed is fixed at 1/200 sec, which is more effective in preventing subject
blur and camera shake than with [1/200-1/60sec. auto]. However, in low light, the
subject's background will come out darker than with [1/200-1/60sec. auto].
Caution
To use slow-sync shooting in < > or < > mode, set to [1/200-30sec. auto].
216
> mode or < > mode.