This area needs more publications to demonstrate its essence and boundary. I am tired of being asked “what is the difference between ad-hoc entity retrieval and (knowledge based) question answering ?” or “Is what you have done similar to Siri that can answer users’ questions ?”
I emphasized the term “structure-aware” in my previous papers on entity retrieval. From my perspective, a typical entity retrieval framework shall degenerate into a document retrieval model if it has no structure to utilize.
The research on entity retrieval is, to some extent, the research on document retrieval in the context of a certain kind of knowledge structure (e.g. knowledge graph, rdf, wikipedia articles …).