We link two references here. The first links to the official definitions by the NIAC and the second one links to latest FAIRmat definitions.
The FAIRmat definitions are regularly contributed to NIAC (around every 6 months), but generally reflect a state which is still under development and may contain new or improved application definitions or base classes. Consider it as the public review stage of these application definitions. However, there might be some parts which are still under discussion and will be subject to change.
Note: To connect NeXus concepts with semantic web tools, efforts are underway to represent them using the W3C Web Ontology Language (OWL). See the NeXusOntology for more details.