This option controls flash sync speed.
Fixing Shutter Speed at the Flash Sync Speed Limit
To fix shutter speed at the sync speed limit in shutter-priority auto or manual exposure modes, select the next shutter speed after the slowest possible shutter speed (30 s or %). An X (flash sync indicator) will be displayed in the viewfinder and control panel together with the flash sync speed.
Auto FP High-Speed Sync
Auto FP high-speed sync allows the flash to be used at the highest shutter speed supported by the camera, making it possible to choose the maximum aperture for reduced depth of field even when the subject is backlit in bright sunlight. The information display flash mode indicator shows “FP” when auto FP high-speed sync is active.