The raw EMG data is really just a voltage that is being read by your Arduino's Analog to Digital converter. The only difference between the RAW output and the SIG output is the RAW output is essentially an AC voltage so it can swing from positive to negative voltages where the SIG output is rectified to only go from 0-Vs. Page six of the MyoWare User Manual goes over this in some more detail with some graphs showing the difference. Also note that you cannot use the Gain Potentiometer to adjust the RAW output.
I hope this answers your question thoroughly. Reply to this post if you have any other questions about the MyoWare Sensor and we would be happy to help as much as we can.