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Each Display has a Display Number that's set in the Display Settings (right-click, select Display Settings). By default it is Zero, so you have to manually change it for every Display.
That Display Number can be used with the PLC in two ways:
1. The PanelView writes the Display Number to a Tag in the PLC so that the PLC knows what screen you're on.
2. The PanelView reads a Tag from the PLC and navigates to that Display Number when the number is nonzero.
Both of these features are configured in the Global Connections section of the Project Organizer.
"Replace Display Number" is the Tag to which the PanelView writes the currently open Display number.
"Remote Display Number" is the tag that the PanelView reads from the PLC in order to change displays when the value is nonzero.