In Perspective and Camera viewports, this flyout lets you choose between Walkthrough navigation, 2D Pan Zoom, or Pan/Truck.
The use of the Pan or Truck button depends on which kind of viewport you are in:
2D Pan Zoom mode lets you pan or zoom a viewport without changing the rendering frame. (Nitrous viewports only.)
The Walk Through button is one way to turn on walkthrough navigation.
This flyout doesn't appear for orthographic viewports or spotlight viewports. Spotlight viewports don't provide walkthrough navigation, and show the Truck and 2D Pan Zoom flyout. Orthographic viewports show only the Pan button.