In OData V4 we have added the concept of fetching related references through appending /$ref to the navigation. This returns a result semantically more similar to RDF, which returns ids of related elements.

Some services, particularly in constrained environments, may be only be able to return the keys of related references. These services push the burden of the work on the client to build the graph from the individual pieces.

In other cases, especially for small nested/contained results, the service might only be able to represent the related elements as expanded relationships.

We already support both of these representations, and having services return them in this format (even without an expand) is legal, but we don't have a way to tell the client that the service does this by default and may not support expand.

We should recognize such services exist and provide a way to tell clients of this behavior.