Serve as metadata intermediaries between information objects and access systems.

• Data Space Transfer Protocol (DSTP) - used to index and categorize data (e.g. file, type of file, database, or distributed database) using an XML-based catalogue which contains UCK (Universal Correlation Key) tags that act as identification keys
• MIX project (Mediation of Information using XML) - National Partnership for Advanced Informational Infrastrucutre (NPACI)
• Universal Prevervation Format (UPF)
• The Warwick Framework: A Container Architecture for Diverse Sets of Metadata - Carl Lagoze, D-Lib Magazine, July/August 1996