ATG Data anywhere architecture provide capability to map multiple data-store to a single repository. Consider a scenario in which you have only the user-id in the relational database and rest of attributes like name and address are in LDAP server. ATG composite repository will help you in this situation.
How to create ATG secure repository?
Refer this post to write a secure repository.
ATG Composite Repository Component Diagram.
Refer Oracle official documentation for more details on ATG Composite repository.