Activate this option when you want to override the “lock” on the geometry Normals in your existing Maya scene during the import process.
Use this option only when you import geometry into an existing Maya scene.
In Maya, you can lock geometry Normals. Once you lock them, the plug-ins consider them user defined, and the application no longer automatically calculates them. This option makes the importer ignore these locks and update the geometry Normals whether you lock them or not.
