Either " normal " or " bbox ".
The navigate command controls the appearance of objects as they are interactively rotated, panned, or zoomed. When the navigation type is normal , operations (rotations, pans, and zooms) will involve the entire object with its existing appearance. When the navigation type is bbox , operations will involve a bounding box which encompasses the object. This allows operations to proceed much more quickly at the loss of visual detail. Once the operation stops, the object returns to its pre-operation appearance.