Mixing : Basic Mixing : Audio Input and Output Paths : Inactive Outputs

Inactive Outputs
An output path can be made inactive, either globally in the I/O Setup dialog (affecting all tracks assigned to that path), or locally per assignment (only affecting that output on that audio track Auxiliary Input track, or Instrument track).
To make an output path inactive, do one of the following:
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Command-Control-click (Mac) or Control-Start-click (Windows) the Output selector.
– or – 
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When opening sessions, outputs will be made inactive automatically if the required hardware or other resources are not available.
Inactive outputs retain all associated automation playlists. Edits made in the session also affect the inactive track’s output automation playlists.
To make an inactive output path active, do one of the following:
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Command-Control-click (Mac) or Control-Start-click (Windows) the Output selector.
– or – 
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