The RKD Search Graph is a search-index-friendly, flattened representation of the RKD Knowledge Graph. It uses the https://data.rkd.nl/search# namespace to represent field names. It's not very semantically meaningful; for a true linked data representation, please visit the Knowledge Graph.
The Search Graph also contains the RKD 'synopses', snippets of html with thombstone information about the RKD's records.