Metacat uses XML for data exchange between the client application and the server.

XSLT is used for transformations for visual presentation (e.g. XML->HTML). It can also be used to tranlate between different types of heterogeneous XML documents which are symantically similar.

Metacat can be set on the fly to either transform or not transform a resultset from XML. This is achieved by setting the 'qformat' servlet parameter to 'html' or 'xml'. When qformat is set to html, Metacat will transform the resultset based on the stylesheet specified in the resultStyleURL of the Metacat Properties file. This XSL stylesheet defines the rules for how to translate the XML tags into HTML tags.

A user can specify that a certain doctype is always transformed using a specific stylesheet by registering that stylesheet in the Metacat Catalog. The Catalog is stored in the database table xml_catalog and contains a list of doctypes and the stylesheets used to transform them. If a doctype has an assigned stylesheet, whenever a document of that type is "read" from Metacat and qformat is set to HTML, the document will be transformed using the assigned stylesheet.