It is a definition. Whether it is normative or not doesn’t really matter. The thing that matters is that other RDF implementations may not be able to use it. So, what about just adding the following sentence (adapted from RDF Concepts): “Note that the use of generalized RDF datasets may cause interoperability problems and that there is no requirement on the part of any RDF tool to accept, process, or produce anything beyond regular RDF triples, graphs, and datasets.”