In a MIDI editor window, the Solo and Mute buttons affect tracks in all windows. They let you either audition the tracks shown in the current MIDI Editor window without the rest of the mix, or let you hear just the mix without the tracks shown in the MIDI Editor window.
The Solo button lets you solo all the tracks currently displayed in the MIDI Editor window.
The Mute button lets you mute all the tracks currently displayed in the MIDI Editor window.
The Solo and Mute buttons are global controls in that they affect all tracks shown in a MIDI Editor window. If you independently solo or mute tracks in the Edit window that are also shown in a MIDI Editor window, the appearance of the Solo and Mute buttons updates to indicate mixed states. Mixed solo and mute states occur when one or more tracks shown in the MIDI Editor window, but not all, are soloed or muted.