In a nutshell, the new XML Catalog features allow you to easily configure TeXMLMate to substitute local resources (like DTDs and other schemas) for remote resources referenced by URLs. This allows, for example, convenient validation of XHTML documents against a local (possibly customized) copy of an XHTML DTD without having to change the URLs used in the XHTML doctype to local file paths. Parsing/validation with local schemas is much faster and allows offline validation.