The name Excellon format is derived from the Excellon Automation Company, which was the market leader in PCB drilling and routing machines during the 1980s, and whose proprietary format became widely used.
The Excellon format was designed for and is primarily used to drive CNC drill and route machines. It is suitable for that task as it can specify machine specific information such drill feed and speed.
The Excellon format is also used to transfer design information from CAD to CAM. It is not adequate for that task: essential information such as plating and drill span is missing. Furthermore, the Excellon output in CAD systems is often poorly implemented, resulting in poor registration between drill holes and copper layers and other problems. For data exchange between CAD/CAM systems it is more practical to describe the drill data in Gerber format as the quality of the Gerber file output software is usually better and Gerber supports attributes to transfer meta-information such as which are the via holes.
The XNC format
The XNC format is strict subset of the IPC-NC-349 specification targeted at data exchange between CAD and CAM. The name XNC format stands for Exchange NC format. As a strict subset, it is highly compatible with existing software. The specification is brief and very detailed, with the purpose of being easy to understand and unambiguous. 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. The specification is freely available at the Ucamco download page.  
M48 Start of header METRIC Metric units (mm) T01C0.6 Tool 1 has diameter 0.6mm T02C1.0 Tool 2 has diameter 1.0mm % End of header G05 Set drill mode T01 Select tool 1 X8.5Y4.8 Drill a hole of 0.6 mm at coordinates 8.5mm,4.8mm X8.55Y2.85 Drill X6.54Y2.85 Drill X6.45Y4.8 Drill T02 Select tool 2 G00X10.25Y3.825 Move to coordinates 10.25mm,3.825mm M15 Plunge rout tool down G01X6.50Y3.25 Rout to coordinates 6.5mm,3.25mm M16 Lift rout tool up M30 End of file
The name Excellon format or Excellon file is also commonly used for files that only vaguely follow any specification. These files contain a few Excellon commands, but follow neither the Excellon nor the IPC-NC-349 specification. It would be more appropriate to call them Excellon-like or generic NC files. Commands are not used properly, or are used in a syntactically incorrect way, and binary data objects may be included. Sometimes the historic EIA or EBCDIC character encoding is used instead of ASCII as required by the standards. Usually the header is incomplete: essential information such as the scale or the tool diameters is missing. Sometimes there is no header at all, and the file only contains tool numbers, with an unspecified diameter, and X,Y coordinates, in an unspecified unit. An example:
% T01 X006272Y001092 X006354Y001093 X006653Y001092 ... T02 X008091Y001754 X-002028 M30
These files are meaningless without additional information, typically put in a free format human readable tool file. This information must be re-entered manually by the CAD/CAM operator. When critical information is missing (e.g. omitting leading or trailing zeroes without properly documenting if and which zeroes are omitted) the file is ambiguous. Still, such Excellon-like files are sometimes used, resulting in unnecessary manual labor and risk of delays or errors. The operator may substitute his own judgement or stop the job pending clarification.
- "Archived copy". Archived from the original on 2007-10-30. Retrieved 2007-10-30.CS1 maint: Archived copy as title (link) original
- IPC-NC-349 Computer Numerical Control Formatting for Drillers and Routers, published in 1985
- The RS-274X Format, published by Ucamco in December 2010, downloadable at http://www.ucamco.com
- "PCB Layout Data". Eurocircuits. Archived from the original on 6 December 2011. Retrieved 26 November 2011.
- Tavernier, Karel. "PCB Fabrication Data - A Guide - section on drill files". Retrieved 16 January 2015.
- "CAD software firms develops XNC format for PCB drill date". pcdandf. Retrieved 26 February 2019.
- "XNC file format specification". Ucamco. Retrieved 26 February 2019.