There are several inconsistencies in netbeans xml catalog representation:

org.netbeans.modules.xml.catalog.CatalogEntry - is a wrapper for the externally defined CatalogReaders.
1. But CatalogEntry in some cases use CatalogReader to get systemId (see CatalogEntry#getSystemId) in some cases don't
use: (see CatalogEntry#getSystemIdValue())
2. CatalogEntry uses systemId as an URI of the resource location, but it is not declared anywhere. At the same time
CatalogReader provides resolveURI method. Similar regarding CatalogEntryNode#getCookie - constructing ViewCookie the
systemId is used as an URI.

3. org.netbeans.modules.xml.catalog.spi.CatalogReader doesn't provide clear javadoc which is results in absolutely
inconsistent implementations which is the source of the problem with using registered catalogs (for example - issue
http://www.netbeans.org/issues/show_bug.cgi?id=163721 ...)

4. there are different approaches in using SCHEMA: or SYSTEM: prefixes in CatalogReaders and CatalogEntry.
Some catalogs don't use it at all (org.netbeans.modules.websvc.core.jaxws.WebServicesSchemaCatalog), some use it as
publicId(org.netbeans.modules.j2ee.persistence.unit.PersistenceCatalog),CatalogEntry removes these prefixes to define
systemId...