You choose what you need to hear and adjust as needed during performance. Singers, for example, need to control their own volume, but will also need to hear whatever instruments they rely on for a pitch reference. So Input 1 might be an aux mix including only their vocal—or a mix of all vocals with their voice loudest; Input 2 could have a mix of guitar and keyboards, and Input 3 could be a mix of bass and kick drum. This is just one example!