The NC formats were originally designed by CNC drill and route machine vendors as proprietary input formats for their equipment, and are known under their company name: Excellon, Hitachi, Sieb & Meyer, Posalux, etc.
[1] [2] The NC files are primarily used to drive CNC machines, and they are adequate for that task.
Many indefinite NC files pick some elements of the IPC standard.
[1] A digital rights managed copy of the specification is available from the IPC website, for a fee.
The XNC format is strict subset of the IPC-NC-349 specification targeted at data exchange between CAD and CAM.
Its purpose is to address the current chaos of different subsets and incomplete NC files, and define a simple common standard.
The XNC subset was defined by a consortium formed by Graphicode, Ucamco, KiCad and Pentalogix.
[8] The specification is ambiguous, leading to different interpretations, especially in the area of coordinates and units.
Commands are not used properly, or are used in a syntactically incorrect way, and binary data objects may be included.