The package is available for licensing under different licenses:differences between various licenses.
Middleware License is a special kind of license, usually needed when you develop some component or library "for sale" (i.e. for licensing to other software developers) and use CallbackRegistry as a part of your component or library. If you are interested in getting more details or purchasing Middleware license, you are welcome to request information about licensing terms. Be sure to specify your planned use of CallbackRegistry and other details that might be of use in negotiations.