debian mentors/ glossary/ DFSG
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Debian Free Software Guidelines

The Debian Free Software Guidelines (DFSG) defines what Debian believes is Free Software.

It is a kind of agreement with the users of Debian as to what they can expect to do with the software that Debian packages for them.

For package maintainers it's most useful function is to define what can go in to main, the Debian distribution. If the license your package is under doesn't meet the DFSG then it cannot be distributed in main. (See Licenses for more on this).