STL files are only used for visual reference within Fusion.
Also note that STL files have surface geometry that is converted for 3D printing into triangular regions, often referred to as facets. For this reason they are not considered worthwhile to use for any kind of CAD/CAM that involves accuracy.
Regards,
Phil Eichmiller
Autodesk, Inc.
Phil Eichmiller
Software Engineer
Quality Assurance
Autodesk, Inc.