From DeveloperWorks, Developing integration solutions with WebSphere Process Server relationships
The Relationship Service in IBM WebSphere Process Server is an infrastructure service that is well integrated with other transformation components. It includes a comprehensive administrative interface that you can use to easily implement complex integration scenarios without custom development of cross-referencing logic mixed in with your business logic. This article introduces the capabilities of the WebSphere Process Server Relationship Service, including those new in V6.1, and explains when and how to use these capabilities. (IBM WebSphere Developer Technical Journal)
To be honest, I’ve never used the relationship runtime and I haven’t encountered a client who has either. I just get the feeling that is has a lot of ‘moving parts’ so little use that recommending it would result in more problem reports than the functionality is worth.