Hi @Bob_Weinstein,
Device Volume — Causes Roon to pass volume commands on to your device via the device’s driver. Exactly what happens next varies from device to device. If your device advertises a high-quality volume control over USB, it’s likely that this is the setting that you want.
Fixed Volume — Disables volume control from within Roon, sending a fixed-level output signal to the audio device. Use this setting if you plan to control volume elsewhere–for instance, on your pre-amplifier or AV receiver.