Sync and Video : Working with Video in Pro Tools : Video Track Controls and Indicators : Video Online Button

Video Online Button
The Video Online button lets you select the main video track, switch that track online or offline, and see at a glance by color whether a video track is offline (gray) or online (blue).
 
To switch a video track online or offline:
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Click the Video Online button of any track that is offline (gray) to put it online (blue), making that track the main video track.
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Click the Video Online button of the main video track to take it offline, and Pro Tools will not output video.