Points > Crop (applicable to any point object) deletes all points except selected points. This feature is useful for cleaning up multiple, registered scans of a physical object. Edit > Select > Best Data selects the best data in a region where two scans overlap, as well as all data that has only a single source.
To remove all points beyond the current selection:
Click Points > Crop.
To select the best points and remove the unwanted points:
Click Edit > Select > Best Data.
Click Points > Crop.