The top group, being the native NURBS, will contain the exact mathematical representation of the model data.. Your CAM package will process a toolpath from that data (Essentially dicing it up) per it's particular settings and CAM tolerances etc...

The bottom group will be your model "Diced up on export", into all "straight facets/edges" etc.... Your CAM package will have to read that, and also try to apply it's tolerances etc, to the already diced up model...

It will be preferred for you to have the NURBS data in the CAM package.