Pliant is copyrighted by Hubert Tonneau, contains a few external contributions (see source code), and is released under the GNU public license, version 2.
In very few words, it means that if you want to use Pliant for writing an application for your company or organization, then you have no special constrains, but if you want to release or sell the application to other companies or organizations, you have to release it under GNU public license version 2, which in very few words means that it has to be free.
Please keep in mind that from a technical point of view, Pliant is a dynamic compiler (the application runs from the source code), so if you intend to write an application and keep the source code secret because you think it's valuable for your business, then Pliant is not suited for you from the technical point of view so that you don't mind the restrictions of the license.
There are also a few applications called open source that are proprietary (as opposed to free) but released with source code. Should you want to write such an application in Pliant, and release it, then you would need Hubert Tonneau to release Pliant to you under a less restrictive license than GNU public license version 2. If that is what you expect, please contact Hubert Tonneau before starting your project.
Pliant development model and license planned evolution
It is not very clear whether the best license for Pliant is GPL or the less restrictive LGPL.
Over it's first 10 years of existence, Pliant has not spread much if at all. Reasons are: too different, not documented, not seriously marketed, etc. Another important reason is that I was more interested in making my computing visions progress, I mean moving forward on the concepts, than spreading already implemented parts. Many external contributors would have meant spending more time coordinating than thinking, and more progress at features than concepts level.
These 10 years have been just enough for me to understand what I'm good and not to work at and so decide a suitable organization for Pliant taking off.
There comes the final organization from: I'm not targeting Pliant for end users or end developers, but rather for people that want to start something new (an application or a product or a company) in a year or so. So external contributors are welcome to fill the gap with end users needs, either through concentrating on proper packaging, or features enhancing of one part, or providing more focused application framework, or providing high quality courses, etc.
Now, if the GPL license is not suited for enabling some Pliant contributors to be fairly rewarded, then I will provide the extra license suited for each particular case, and if at some point the fact that I decide it seems abusive, then it will be time to create a foundation.