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The Debian Free Software Guidelines (DFSG) is a part of the Debian SocialContract. An incomplete collection of DFSG-compatible licenses is at DFSGLicenses.

Debian Policy Repackaged upstream source describes when and why Debian packages are repackaged to fit dfsg, rather than shipping a pristine source tarball.
The uscan utility (from the Debian descripts package) and gbp import-orig (from git-buildpackage) may be used to create dfsg source tarballs.