Taxonomies or sets of controlled vocabularies are typically implemented as values for various metadata elements (also called metadata properties or fields). Metadata elements that contain controlled vocabularies could be Topic, Activity, Location, Organization Name, People/Role Type, Document Type, Content Language, etc. These are often implemented as facets in faceted search, although they do not have to be. There may be additional metadata elements for non-taxonomy values, such as Document Title, Image Caption, Creator/Author, Creation Date, Rights Status, etc. In addition to designing taxonomies, in my consulting projects I often also design such broader metadata schemas.

Custom Metadata Schemas

A custom (use case-specific) metadata schema specifies which metadata elements to include for different purposes. These include content tagging and management, content workflow management, end-user search filters, or merely displayed on content records for identification.

A custom metadata schema may specify the following:

• A definition for each metadata element
• Sample values for each metadata element
• In what user interfaces the metadata element appears
• The ownership or authority of a metadata element, whether a department or role

A custom metadata schema also specifies rules about the application of each metadata element, including:

• The value type for the metadata element (For example, controlled vocabulary terms, uncontrolled keywords, free text, date, integers, Boolean yes/no, etc.)
• Whether assignment of a value from the metadata element is required or optional for each content item (or depends on the specific type of content item).
• Whether the assignment of the values from the metadata element is limited to just one or can be multiple, which is referred to as “cardinality.” (For example, the assignment of only one Document Type but up to four Topics per content item.)

Standard Metadata Element Sets and Schemas

In the context of metadata schemas, there exist not only these custom metadata schemas, but also *standard *metadata sets of elements and their schemas. They provide predefined metadata elements that are intended to be sufficiently generic for various use cases. Perhaps the most widely used standard metadata schema in is Dublin Core, which is a set of 15 basic (core) elements intended for published documents. These elements are Title, Subject, Description, Type, Source, Relations, Coverage, Creator, Publisher, Contributor, Rights, Date, Format, Identifier, and Language. There are other standard metadata schema that are somewhat more specific for a subject domain, such as IPTC (International Press Telecommunications Council) metadata which is intended for images. When standard data notation, such as XML or RDF, whose specification may also part of the standard metadata scheme, metadata can then be shared.

Standard metadata schema include information for each element such as definition and type, but unlike custom metadata schemas, standard metadata schemas do not include any instructions on their application, such as cardinality and implementation, as that depends on each use case. Therefore, if you choose to apply a standard metadata schema, you need to additionally decide and document how it should be applied, especially which elements are to be used for which purposes, in which systems, along with metadata element-specific rules of requirements and cardinality, as describe above. This kind of document is referred to as an application profile.

My most recent conference presentation, a panel at the DCMI (Dublin Core Metadata Initiative) conference in Barcelona, October 22-25, addressed application profiles. Panel organizer, Joseph Busch, explained in his presentation: “An application profile defines a specific set of requirements, settings, and metadata for a particular application to ensure compatibility and functionality. The profile adapts general standards or frameworks to meet the needs of a specific use case, for example.”

Taxonomists usually don’t speak to their stakeholders or clients of “application profiles,” because such specifications are typically already included within a larger* taxonomy governance plan*, something taxonomists commonly create and promote. When taxonomists work specifically with metadata experts, however, they should consider the specific needs of an application profile.

Finally, a standard metadata schema, with its predefined labels for metadata elements, can also be considered a kind of (controlled) vocabulary. This is the topic of my next blog post, “Schema and Value Vocabularies.”