Contains entities, concepts and synonyms.
A dataset in the Open Numbers namespace MUST use the entities and concepts and their properties defined in this ontology if its data has identical semantics.
If an identical concept, entity or property cannot be found in the ontology, you MAY use other entities, concepts and properties or you MAY leave out the the data from the dataset.
Identity in the Open Numbers namespace is quite loosely defined to allow for comparison of many data sources. Identical concepts measure the same phenomenon but don't necessarily need to use the same method of measurement.
Synonyms contain entity and concept ids from the ontology with often found synonyms which can be used for semantic harmonization (i.e. translating a dataset from one namespace to another).