Sessions & Tracks : Tracks : Soloing and Muting Tracks : Solo Safe Mode

Solo Safe Mode
Pro Tools lets you solo safe a track. This prevents the track from being muted even if you solo other tracks. This feature is useful for tracks such as Auxiliary Inputs that are being used as a submix of audio tracks, or effects returns, allowing the audio or effects track to remain in a mix even when other tracks are soloed. It is also useful to solo safe MIDI tracks so that their playback is not affected when you solo audio tracks.
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AFL or PFL soloed tracks (Pro Tools HD only) cannot be solo safed.
To solo safe a track:
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Control-click (Windows) or Command-click (Mac) the Solo button on the track. This prevents the track from being muted even if you solo other tracks. The Solo button changes to a transparent color in Solo Safe mode.
To return a solo safe track to normal:
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Control-click (Windows) or Command-click (Mac) the Solo button on the track again.