Due to constraints in the JAAS specification, one class has to be available for all bundles. This class is called ProxyLoginModule and is a LoginModule that acts as a proxy for an OSGi defined LoginModule. If you plan to integrate this feature into another OSGi runtime, this class must be made available from the system classloader and the related package part of the boot delegation classpath (or be deployed as a fragment attached to the system bundle).
The xml schema defined above allows the use of a simple xml (leveraging spring xml extensibility) to configure and register a JAAS configuration for a given realm. This configuration will be made available into the OSGi registry as a JaasRealm and the OSGi specific configuration will look for such services. Then the proxy login module will be able to use the information provided by the realm to actually load the class from the bundle containing the real login module.