There is only one file type that uses the .XNC extension:
Exchange NC File
An XNC file is a text file saved in the Exchange NC (XNC) format. It contains numerical control instructions (XY coordinates) for directing a machine tool, such as a drill, for the production of Printed Circuit Boards (PCB). XNC files are designed for exchanging information between CAD and CAM programs.
Updated: February 13, 2019