The miniball software is available under the GNU General Public License (GPLv3 - https://www.gnu.org/copyleft/gpl.html).
If your intended use is not compliant with this license, please buy a commercial license (EUR 500 - https://people.inf.ethz.ch/gaertner/subdir/software/miniball/license.html).
You need a license if the software that you develop using Miniball V3.0 is not open source.