System Configuration : I/O Setup : Editing Paths : Making Paths Active or Inactive

Making Paths Active or Inactive
Pro Tools paths can be Active (on) or Inactive (off, or unavailable). Paths can be globally activated or deactivated in the I/O Setup. Making a signal path inactive will turn off the signal path on any and all tracks currently assigned to it.
Pro Tools also sets unavailable paths to inactive. Paths can be unavailable when hardware or other system resources are unavailable, such as when opening a session saved on a different system.
Tracks can also be made active or inactive. For information, see Making Track Inputs and Outputs Inactive from the Edit or Mix Window.
To globally activate or deactivate a path:
1 Choose Setup > I/O.
2 Select a path type using the tabs at the top of the window.
3 Set the Active/Inactive control for the path.
 
Any tracks that were assigned to the now inactive path will show that path's name in italics on the track's I/O selectors.
4 Click OK to save changes and close the I/O Setup. Path status is displayed as follows:
Italics
Indicates the path is inactive.
Non-Italics
Indicates the path is active.
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With overlapping output paths of different channel widths, if the widest path is made inactive, all other overlapped output paths will not pass audio from Pro Tools.