If you set Custom Setting e1 Flash Sync Speed between 1/60th sec. and 1/250th sec. the maximum shutter speed the camera will use in P, or A exposure modes will be the speed you set in e1. FP high speed sync mode will be disabled.
If you set e1 to 1/250s (Auto FP) when using an SB-600, SB-700, SB-800, SB-900, SB-910, when the shutter speed is faster than 1/250th sec. Auto FP High Speed Sync mode will be activated.