Bounding Box displays an axis-aligned box that fully encloses the selected object or objects. You can find it under the Inspect tab.
When Bounding Box is enabled, MeshInspector calculates the smallest rectangular volume that contains the entire geometry and draws it in the viewport.
Dense BBox, or Dense Bounding Box, displays a tighter, geometry-aware bounding box that more closely follows the actual shape of the object. The tool can be found, as Show Dense BBox, under the same Inspect tab.
Unlike the Bounding Box, Dense BBox is not limited to a simple axis-aligned extent based only on extreme points. Instead, it analyzes the object’s geometry in more detail to produce a minimal enclosing box.
Note that Bounding Box and Dense BBox can be active simultaneously.
How to Use the Bounding Box and Dense Bounding Box Tools