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.
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.