OpenTag is open source software that is licensed primarily by the OpenTag License Version 1.0. Some parts of the Mainline Distribution are additionally licensed using the BSD License, which is compatible with the OpenTag License.
Most OpenTag code is licensed via the OpenTag License. Certain application-level code is licensed under BSD, in order to emphasize that the application is extricable from the OpenTag core, and also to provide the most common open source license (BSD) to code that is not necessarily intrinsic to OpenTag.
You may maintain your own distribution of OpenTag, combining core code and apps, that includes code licensed under any license you choose. Code in the Mainline Distribution must be licensed via the OpenTag or BSD licenses. Therefore, if you wish for your components to be included in the Mainline Distribution, you must use OpenTag or BSD licenses.
If you are a commercial product developer, and you are using OpenTag, you are free to create new code modules using any license you choose. You must abide by the requirements of the licenses in the code you are using, although both BSD and OpenTag licenses are permissive, and they may be statically linked with commercial/proprietary licensed code in such a way that the total package is subject to the commercial/proprietary license as long as the BSD and OpenTag parts are noted in the full product distribution and made available independently as open source. Neither BSD licensed code (implicit) nor OpenTag licensed code (explicit) grant intellectual property licenses to IP that may be exemplified in the distribution, so be sure to contact the provider of the distribution before deploying your product commercially.
The OpenTag License Version 1.0 is an open source license, with similarities to the BSD license, but with explicit terms related to licensing of intellectual property. The OpenTag License Version 1.0 also draws from certain elements of the Apache License Version 2.0.
As of 2012, legal proceedings in the United States and recent legislation in the EU have lead to the precedent (or law) that existing open source licenses do not guarantee royalty-free usage in the presence of related intellectual property. The OpenTag License explicitly applies said US and EU rulings to its worldwide user-base, putting everyone on equal ground. Additionally, the OpenTag License stipulates that enforcement of any potential royalties must be RAND (Reasonable And Non-Discriminatory).