The heart of the registry is the data element; a basic unit of information collected about anything of interest—for example, a pharmaceutical name or the city in which a patient lives. The data in the registry is not the drug name itself or the actual city name, but rather the metadata about how information is collected. Metadata is often defined as "data about data", a definition which is technically correct, but does not convey the richness of data information which must be recorded. In order to use a piece of data, one must know specific things precisely placing the data item in terms of meaning, quality, context, chronology, and source. The specific things—metadata—are expressed in the form of attributes.
The content of the attributes for each data element is found at the most specific level of the registry on the Data Element Details page. Not all attributes may be collected by each organization for each data element, although there is a subset, which is generally considered best practice and, for standards purposes, is required. Data elements within the Registry are documented within a standard format, using the ISO/IEC international standard 11179, "Information technology — Metadata Registries — Part 3, Registry Metamodel and basic attributes." According to the registry metamodel standard, a data element is essentially a unit of data for which the definition, identification, representation, and permissible values are specified by means of a set of attributes.