NWC is a cache file. “When you open a CAD file in Navisworks, by default, a corresponding cache file (NWC) is created, which contains all of the conversion details required by Navisworks. When you subsequently open that CAD file in Navisworks, it will check to see whether a cache file is available. If it is, then Navisworks will check to see whether the CAD file has been modified since it was last opened in Navisworks.” 
 The Massachusetts Port Authority, 2015. BIM Guidelines for Vertical and Horizontal Construction.