The document linked below presents the results of a process to develop exemplary qualifiers for the Dublin Core Metadata Element Set (DCMES). The qualifiers listed were identified in element-specific working groups of the Dublin Core Metadata Initiative (DCMI) and judged by the DCMI Usage Committee to be in conformance with principles of good practice for the qualification of Dublin Core metadata elements. Working groups proposed candidate qualifiers judged likely to be of widespread use (and hence to increase the likelihood of interoperability among applications) and these proposals were subsequently debated and balloted by the DCMI Usage Committee.
In determining the makeup of these qualifiers, preference was given to vocabularies, notations, and terms already maintained by established agencies. It should be emphasized that the list of externally-maintained vocabularies identified is a preliminary list. There are many more controlled vocabularies or classification systems that are not identified here. The DCMI welcomes suggestions concerning additional existing standards that might serve as qualifiers.
Inevitably, there will be situations where an agent or client will encounter DCMES descriptions that use unfamiliar qualifiers developed by implementors for specialized local or domain-specific needs. The useful interpretation of such a DCMES description will depend on the ability of an application to ignore the unknown qualifiers and fall back on the broader meaning of the element in its unqualified form. The guiding principle for the qualification of Dublin Core elements, colloquially known as the Dumb-Down Principle, is that a client should be able to ignore any qualifier and use the description as if it were unqualified. While this may result in some loss of specificity, the remaining element value (without the qualifier) should continue to be generally correct and useful for discovery.
It is expected that implementors will develop additional qualifiers for use within local applications or specific domains. Such qualifiers may not be understood by other applications. However, qualifiers that conform to the principles of qualification defined are more likely to be reusable by other communities within the broader context of cross-domain discovery.
At the time of the ratification of this document, the DCMI recognizes two broad classes of qualifiers:

• Element Refinement. These qualifiers make the meaning of an element narrower or more specific. A refined element shares the meaning of the unqualified element, but with a more restricted scope. A client that does not understand a specific element refinement term should be able to ignore the qualifier and treat the metadata value as if it were an unqualified (broader) element. The definitions of element refinement terms for qualifiers must be publicly available.
• Encoding Scheme. These qualifiers identify schemes that aid in the interpretation of an element value. These schemes include controlled vocabularies and formal notations or parsing rules. A value expressed using an encoding scheme will thus be a token selected from a controlled vocabulary (e.g., a term from a classification system or set of subject headings) or a string formatted in accordance with a formal notation (e.g., "2000-01-01" as the standard expression of a date). If an encoding scheme is not understood by a client or agent, the value may still be useful to a human reader. The definitive description of an encoding scheme for qualifiers must be clearly identified and available for public use.

All of the qualifiers listed fall into one of these two categories. Additional qualifier categories may evolve over time and with implementation experience. Proposed qualifiers which appeared to the DCMI Usage Committee to fall outside of these categories, or which fell short in some other respect (e.g., clarity of definition), will become the focus of further discussion and be reconsidered for approval in the future.
The qualifiers listed do not constitute a closed set, designed to meet all of the descriptive needs of implementors. Rather, they form the foundation for a larger body of qualifiers that will evolve as additional qualifiers are developed by various communities and submitted to the DCMI Usage Committee for review and approval. Implementors may deploy these qualifiers with confidence that they conform to the Dumb-Down Principle, and are encouraged to use these qualifiers as examples to guide development of local qualifiers for Dublin Core metadata elements.

For more information from the original document, see:

http://dublincore.org/documents/2000/07/11/dcmes-qualifiers/