pappl-retrofit is developed by OpenPrinting and distributed as open source software under the Apache License, Version 2.0 with exceptions to allow linking to GPL2/LGPL2 code (same license as for CUPS).
Contributions should be submitted either as pull requests or as attachments (unified diffs) to bug reports on the pappl-retrofit GitHub project at https://github.com/openprinting/pappl-retrofit.