The supported registries such as LDAP, Operating system, federated repositories are good enough for most of the requirements but there could be cases where you might want to implement custom user registry either for better control or because you have some special situation.
The UserRegistry interface is used to implement both the custom registry and the federated repository options for the user account repository. The interface is helpful in situations where the current user and group information exists in some other format. The UserRegistry interface is also used for LocalOS and LDAP registries, for example, all our registries.
WebSphere Application Server V6.1 also provides a plug-in to support any registry by using the custom registry feature. With the custom registry feature, you can configure any user registry that is not made available through the security configuration panels of the WebSphere Application Server V6.1