jangle.org

This registry is intended to help define authoritative identifiers for negotiating requests for specific metadata types in standards such as Jangle or unAPI.

The identifiers themselves arbitrary; while consistency is helpful, it's not mandated.  Also using a term that makes sense for a given format (such as the explicit mime-type or namespace) will be more intuitive.  The important part is to have agreement on exactly what a given identifier stands for.

http://www.loc.gov/MARC21/slim

MARC21 XML, as defined here: MARC 21 XML Official Site at the Library of Congress

application/marc

Binary USMARC/CANMARC, per RFC2220

http://purl.org/dc/elements/1.1/

Dublin Core Element Set, as defined here.