EXG electrodes are individual active electrodes or cables for biopotential measurement that plug in at the inputs marked EXG1-8.  EXG is shorthand for EEG, EMG, ECG and EOG.  These electrode cables support a single monopolar channel each and they have a keyed 2-contact connector that plugs in at the EXG1-8 inputs on the AD box or in a passive headbox that is itself connected to one of the 32 channel / 68-pin connectors A-H on the top of the AD box.