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The PreTeXt Guide

Section 25.4 GNU Free Documentation License

The GNU Free Documentation License (GFDL) is a license designed for documentation of open source computer programs licensed by the GNU Public License (GPL). However, it explicitly mentions textbooks as one possible use. (GNU is a recursive acronym for “GNU’s Not Unix” and is the software project that originally built all the utilities which complement the Linux Kernel to make up an operating system.)
The GFDL is similar to a CC BY-SA license. It allows unlimited copying, forever. Modified versions that are distributed must acknowledge the original contributions and must also carry a GFDL license. So this is a viral license, always. And another example of copyleft.
However, the GFDL does not employ options like a Creative Commons license. More importantly, the GFDL is very explicit about source (“transparent” copies) and derived output (“opaque” copies), and the license applies to both versions. So the GFDL would say PreTeXt source is transparent, and a resulting PDF is opaque, and the license largely treats them identically.
Because the GFDL has the essential characteristics of CC BY-SA, and is so explicit about simultaneously licensing document source authored with a markup language along with output formats, it is our choice for textbook projects.
So in PreTeXt we might go something like
      <holder>Richard Stallman</holder>
      <minilicense>GNU Free Documentation License</minilicense>
The GFDL is also explicit about including the complete license with your document. You can find various places a version formatted for inclusion in a PreTeXt project, including as part of the source for this document.