The 3D distance is the straight-line distance between the nominal and actual location using a 3D right triangle.
The dN is only the distance along the normal vector of the surface.
Under regular circumstances where a measured point is not at the edge of the part, these values should be the same. However, if your point is measured such that the point projects to the edge of the part, the 3D distance may be larger since it is a specific distance away from the nominal surface. See the picture below for an example of d3D compared to dN on the edge of a surface.
In both cases, the user is inspecting against the green nominal surface. The blue point is within the bounds of the surface so it's nominal projection is directly along the normal of the surface. Thus d3D will be equal to dN. In the case of the red point, the user is outside the bounds of the surface so we can only project to the nearest edge. The regular normal direction is still in the up direction. But our 3D distance is greater than dN because we are only considering the component of the direction along the normal when computing dN.
3d distance, dn, feature dro, inspect surface, cam2