There's a pushbutton switch next to the Input 7-10 jack. When set to mono, the signal going into the Input 7-10 jack - or the mono signal coming from loop 6 if there's nothing plugged into Input 7-10 - will be sent to both the left and right sides. If the pushbutton switch is depressed, switching it to stereo, Input 7-10 is treated as a TRS stereo input. This isn't an assignable feature.