Like the Ogre3d graphics engine it uses, Stallion is licensed under the GNU Lesser Public License (LGPL). We encourage everyone to be open with their games projects but we do not require anyone to open source software they dynamically link to Stallion. As such, you are free to use proprietary software with Stallion projects, but be aware that this may limit who can use your work. We encourage everyone to contribute their changes back to Stallion so that less work needs to be done by others in the future.
Artwork stored in the Stallion art repository is to be licensed under any applicable permissive license or placed in the public domain. We recommend the Creative Commons licenses, the GNU Free Documentation license or even the GNU General Public License (GPL) where the meaning of "source code" is, for example, the native file format of the 3d modelling tool used to make the model (in general, the form most suitable for making changes to the work). Clearly, you can use any license you like for work that you have created, but using a permissive license ensures that others will be free to make games that include your work and distribute it to others.