Pioneer's core code and extension modules are licensed under the terms of the GNU General Public License version 3. See licenses/GPL-3.txt for details.
Pioneer's art, music and other assets (including Lua model scripts) are licensed under the terms of the Creative Commons Attribution-ShareAlike 3.0 Unported License. See licenses/CC-BY-SA-3.0.txt for details.
All GPL code contributed to Pioneer belongs to all the authors of Pioneer, and no individual's names appear at the tops of any of the files.
Copyright © 2008-2013 Pioneer Developers. See AUTHORS.txt for details
Using licensed assets
All of the assets (such as textures or scripts) in a model intended for release with Pioneer's core should be available under the same license, or licenses similar enough to be compatible. The model should be released under the same license. This means no mixing of GPL and CC textures in the same model, for example.
Many licenses require attribution, so make careful note of where you get your assets from along with their licenses.
Anything that is 'free for non-commercial use' is unsuitable - this includes -NC licenses such as CC-BY-NC.
Many textures distributed online are available under multiple licenses.
CC-BY-SA-3.0 and compatible
For simplicity you should try to use assets available under CC-BY-SA-3.0 or compatible, the default license for Pioneer's assets. This includes
Models using these assets should be released under CC-BY-SA-3.0.
CC0 or PD
Creative Commons 0 and Public Domain assets can be relicensed when redistributed, and should be relicensed under the CC-BY-SA-3.0
It is possible to use assets under GPLv3 or the 'or any later version' variant of the GPLv2. The model must be licensed under GPL too.