This is a deep chapter...
A STL file can be imported into NX, it will there show up as a faceted body.
A facet body is composed of small triangular areas, facets. Each facet is flat. It might look smooth when shaded but it isn't.
If you section this you will get "millions" of linear "pieces", - One facet = "one linear piece".
It will show up as a shaded body but it should rather be considered a point cloud, it's the points which have been scanned/ measured and the areas between are simply interpolations.
You can if you like edit the display from shaded to "points" to see the cloud.
You can pick on the facets IF you turn on the "Point on face" snap type. (it's off by default) Thereby you can do quite a lot of things, such as draw splines on top of the facet body or measure.
You can split and merge point clouds in NX but you cannot manipulate the shape. Improving the quality is more or less impossible since we don't have the data, imagine measuring 4 points of a circle, in facet mode this would be rectangular. Improving could be some type of softening of the rectangle, but we would then get points in space without correlation to the original circle. - In animation software this is no problem but for reverse engineering it is.
There are a number of tools that can be used for reverse engineering, you can fit curves and sheets , you can draw splines on the facets, sectioning, the rapid surfacing etc etc.
as said, it is a deep chapter.
Regards,
Tomas