A catalog maps namespaces to schema documents.

Its an XML document in the namespace: urn:oasis:names:tc:entity:xmlns:xml:catalog

The spec is at: OASIS's Website

There are two things to be concerned with:

• Mapping URI values that start with "urn:publicid:..."

• Everything else.

Example:

<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog" prefer='public'>   
    <uri name="http://cde.berkeley.edu/~milowski/schemas/example-form/event/200402" 
         uri="event.xsd"/>
    <public publicId="IDN cde.berkeley.edu//milowski//schemas//example-form//event//200402" 
            uri="event.xsd"/>
</catalog>