If you set the maximum sync speed to 1/250th sec. the flash will sync at all shutter speeds up to a maximum of 1/250th sec.
If you set the maximum sync speed to 1/250th sec. FP mode, when using a compatible Speedlight it will sync at shutter speeds up to 1/8000th sec. though at significantly reduced power. FP mode is very useful when you need fill flash while shooting in bright daylight ambient conditions.