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Defines mappings for external resource identifiers.
Sometimes a reference to an external entity/source cannot accessed very easily and it would be handier if we could use a local version instead.
Example html DOCTYPE:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">The URI specified "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" is accessed over 'http' and this will always be slower than using a local version.
We could now define a Catalog entry which maps the publiId "-//W3C//DTD XHTML 1.0 Transitional//EN" to a different URI.
<public publicId="-//W3C//DTD XHTML 1.0 Transitional//EN" uri="xhtml1-transitional.dtd"/>
If we now supply the catalog to our XML Parser, the 'http' call will be substituted by a call to our local version.