XML catalogs provide mapping information that maps an external entity reference in an XML document to the actual location of the document being referenced. You can use a catalog to redirect mappings for an external entity, such as a DTD or XML file, to a different location.
For example, if your XML document is constrained by a DTD that is located on the Internet, and you are working without an Internet connection, you can redirect the reference to a local copy of the DTD. You can then validate your XML document without an Internet connection. Alternatively, you can use XML catalogs to switch between different versions of the DTD.
The IDE automatically bundles many useful DTDs and XML schemas, such as those for working with web frameworks or the NetBeans platform. The DTD and XML Schema Catalogs node of the Runtime window displays all your registered XML catalogs. You can expand any category's subnode to open the DTD or XML schema in the Source Editor. You can simply register your own DTD or XML schema resources or you can create your own XML catalog and register it with the IDE.
For more information on the OASIS XML Catalog specification, see: