There are certain requirements and restrictions on the use or interchange of works that we do not feel impede the essential freedom in our definition. These restrictions are described below.
Apart from these allowed restrictions, the license must not include clauses that limit essential freedoms. Especially, it must not specify any usage restrictions (such as prohibiting commercial use of the work, restricting use depending on political context, etc.).
Attribution protects the integrity of an original work, and provides credit and recognition for authors. A license may therefore require attribution of the author or authors, provided such attribution does not impede normal use of the work. For example, it would not be acceptable for the license to require a significantly more cumbersome method of attribution when a modified version of the licensed text is distributed.
Transmission of freedoms
The license may include a clause, often called copyleft or share-alike, which ensures that derivative works themselves remain free works. To this effect, it can for example require that all derivative works are made available under the same free license as the original.
Protection of freedoms
The license may include clauses that strive to further ensure that the work is a free work: for example, access to source code, or prohibition of technical measures restricting essential freedoms.