On-board studio quality reverb. Reverb gives you a sense of the space that you are in. A dry guitar signal can sound a little dull. Adding the right about of reverb provides the guitar signal with ambience and brings it to life.
This socket allows you connect an external sound source of your choice and can be used for playing backing tracks into your unit. Please not the volume of music/backing track is controlled by the volume on the external sound source itself.