Without metadata, an identifier is of very little value. Metadata, which may be defined in this context as information about an identified referent, provides human beings or machines with the data they need to enable them to make use of that identified referent. Metadata may include names, identifiers, descriptions, types, classifications, locations, times, measurements, relationships and any other kind of information related to a referent.
There are two ways in which every Registration Agency (RA) is bound to deal with metadata as illustrated below:
•An RA will gather input metadata from referent providers (typically, descriptions of the referents and associated rights and policies).
•An RA will need to provide some level of output or service metadata to support DOI system services.
Input metadata will provide some, but not necessarily all, of the service metadata.
In some cases, a metadata declaration will itself be a complete DOI System service.