Shop rights can be valuable in situations where an inventor has not executed an assignment document for a patent and the inventor is either unavailable or refuses to assign the patent. However, a shop right is not a substitute for an assignment. Instead, shop rights are essentially nothing more than a defense to patent infringement for the employer. Some courts refer to it an implied license, while others call it a form of equitable estoppel. In either situation, a shop right is non-exclusive, royalty-free, and personal to the employer.