XLite100SF is specify to use as front light. The integrated sensor monitoring if there is any motion and start/stop the light automatically.
XLite100SR integrated an accelerometer which allow the light to detect if the skateboard is slowing down. Then automatically change from flash mode to the“brake” mode to tell the others that you are slowing down.
The XLite100SF/SR also inherit the feature from original Xlite100: the light is able detect the ambient light intensity/motion and automatically tuning on/off . E.g. when getting into a dark environment the light will be turned on automatically during day time.