The root element of the catalog file is <catalog>. This element may contain any number of entries. A catalog entry for an object is contained within an <entry> element. The xlink:href attribute of the <entry> element is set to the abstract URI of the object. The <entry> element may have three children: <author>, <title>, and <locator>. The <author> and <title> elements are optional. Arboreal does not make use of these elements, but using them makes the catalog file more readable. If they are present, the catalog file may be opened as a master text and (assuming that it has also been loaded as a catalog) used as a table of contents from which documents can be opened directly. The <locator> element is required; its xlink:href attribute is a URI that specifies the actual location of the object. If this URI is a relative URI, Arboreal understands it as relative to the base from which the catalog file was loaded.
The following is an example of a complete catalog entry for an Archimedes primary text:
<entry type="text" xlink:href="001.xml">
<title>De re metallica</title>