How should I use the 8 output terminals?

You can set whether to use the M+6CH mode or the 8CH mode from the OUTPUT SETTING screen.
In M+6CH mode, you can set the output terminal for each track while monitoring the master sound. This is convenient when recording a specific track, or sending a specific track to a separate output device.
In 8CH mode, all tracks can be output from each of 8 outputs. This is useful when recording the maximum of 4 stem mixes while maintaining the total mix balance, or when splitting rhythm sound into 8 monaural signals for output.