The locations for our standard reference electrodes are at the P3 and P4 locations. CMS/DRL refers to Common Mode Sense (CMS) active electrode and Driven Right Leg (DRL) passive electrode.
Emotiv headsets use CMS (P3 or left mastoid for EPOC, left mastoid for Insight) as an absolute voltage reference. DRL (P4 or right mastoid for EPOC, left mastoid for Insight) is a feedback cancellation system to float the reference level on the common mode body potential. These levels are not recorded anywhere, and are not estimated. They are inherent in the measurement as a relative reference point. This is a standard clinical referencing technique. Other methods of referencing can be inferred mathematically.