If you have the DAC option fitted, five digital sources will be listed as well, AES/EBU, optical, USB, network (Roon) and Bluetooth. These correspond to the four connectors on the DIGIN option panel on the rear of the preamp.Level offset adds a fixed amount to the volume whenever this preset is active. Use this when you find that you always have to turn the volume up or down compared to other sources.Mono does what you’d expect: sum left and right together.The two polarity invert buttons invert the polarity (absolute phase) of the left and/or right channel. Inverting one channel is usually not useful, except when playing back vertically cut records which, played back with a modern cartridge, produce out-of-phase left and right. Otherwise, do not invert only one channel.