One of the principal advantages of SOM is that it makes "black box" (or binary) reusability possible. Consequently, SOM classes are frequently packaged and distributed as class libraries. A class library holds the actual implementation of one or more classes and can be dynamically loaded and unloaded as needed by applications. Importantly, class libraries can also be replaced independently of the applications that use them and, provided that the class implementor observes simple SOM guidelines for preserving binary compatibility, can evolve and expand over time.