What is the difference between a Common Reference electrode and a Patient Ground?

The Patient Ground should always be connected, while the use of the Common Reference electrode depends on your reference method.

The Patient Ground must be connected to keep the amplifier in range and to suppress the noise and mains interference (50/60 Hz) in all channels. The Patient Ground connection can be made using a separate lead connected to the GND input. For some accessories, the Patient Ground connection is also available in the UNI 1-32 multi-connector. Consult the instructions for use of the used accessories to check this. If this is the case, either of them can be used. Options to connect the Patient Ground can be found in this article.

The Common Reference input (REF) is optional to use for EEG and HD-EMG measurements. This input is used as a reference for all EEG channels. This means that, if Common Reference mode is configured, all unipolar inputs are referred to this input. The Common Reference input electrode is placed on the subjects body, close to the EEG or HD-EMG electrodes.