Moves the camera (and its target) parallel to the view plane.
This button replaces the Pan button when a camera viewport is active.
For a target camera, dragging the mouse moves the locked camera and target parallel to the camera view. For a free camera, dragging the mouse moves the camera along its X/Y axes.
To constrain trucking of any viewport to a single axis, hold down the SHIFT key. The truck is constrained to the first axis you move while the SHIFT key is down.
To accelerate trucking, hold down the CTRL key.
The camera or spotlight and its target move parallel to the view plane, which is perpendicular to the camera's line of sight.