Using the XML Catalog
The eclipes WTP project provides support for using an XML Catalog based on the OASIS XML Catalog specification. An XML Catalog allows a user to control how resources references are resolved. So for example if a user has one or more XML documents that reference a DTD or XML Schema via a web address (e.g. http://www.example.org/foo.xsd) they may want to map this address to their own local copy of this resource. This way 'catalog aware' tools won't need to go off to the internet to fetch the resource. There's several other ways an XML Catalog can be used as defined by the OASIS XML Catalog specification. If you're an eclipse plugin developer there's also some convenient extension points you can utilize to contribute to the XML Catalog settings.
Editing the XML Catalog Settings
TODO.. describe how to get to XML Catalog settings
Registering XML Schemas