In Spotlight viewports, this flyout lets you choose between Pan navigation or 2D Pan Zoom mode.

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Truck (for Spotlight viewports)
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2D Pan Zoom Mode
2D Pan Zoom mode lets you pan or zoom a viewport without changing the rendering frame. (Nitrous viewports only.)
This flyout doesn't appear for perspective viewports or camera viewports. Those viewports show the Pan Truck and Walkthrough flyout. Orthographic viewports show the Pan button only.