The OASIS XML Catalog document type
The OASIS XML catalog is a document describing a mapping between
external entity references or URI's and locally-cached equivalents. You can read
more about using catalogs in <oXygen/>
here.
A file is considered to be an XML Catalog document when it has the
following namespace: urn:oasis:names:tc:entity:xmlns:xml:catalog
or when its root element name is catalog
.