Schema metadata is an intentional description of extensional instances. The schema intension has a formal logic relationship to the data instances. This is important in ensuring data quality. It also provides a formal basis for systems. Navigational metadata provides information on how to get to an information resource.
Navigational metadata has no formal logic relationship to the data instances.
Associative metadata provides additional information for application assistance. Associative metadata usually does not have a formal logic relationship to data instances although there may be systematic association relationships. CLRC-RAL was early into this field and experimented with several techniques since 1993, currently basing the departmental web on Microsoft ASP technology. Since this is usually structured, managed, high quality data its use might be preferable to authored html pages. The key to the Future of Information Systems is Metadata.