Some believe this to be the ultimate in freedom while others disagree. BSD style licenses often carry a requirement that a certain copyright notice be included in all subsequent versions of a source file. Public Domain Public Domain software has been around for quite sometime. A key difference between GPL and public domain is that public domain software has no limitation against its later use in or as a part of proprietary software. Source code may or may not be available and modification and subsequent redistribution of such software is usually not allowed.
Cost is usually free.
Copyright has been surrendered by the author to the public domain. Restricted Use Software Not very common, but in existence, restricted use software is often distributed freely and at no cost. Such software is characterized by its license preventing commercial use or use by others than a targeted group of users. Source code is rarely available. Copyright retained by the author.