Control Panel VKEY 100 / external keyboard – AUDIO INPUT: SOLO

This function is similar to Audio Input: Mute function.

The difference is that with Audio Input: Mute function you can simultaneously silence several different Audio Inputs in the mixer, while using Audio Input: Solo function, you can only set one channel as Solo.
This stems from the definition of a Solo function:

Solo – a feature that enables sending a selected signal individually to the output. Enabling this feature for the chosen channel mutes all the other input channels. Only the signal for which the Solo feature has been enabled is sent to the outputs.