The interface provided to a user to select a portlet to be added to a page shows the portlets organized in categories to make it easier to find them. The file liferay-display.xml is available to portlet developers to specify how they want their portlets to be categorized. Following is an example of what this file may look like:

<display>
	<category name="category.admin">
		<portlet id="9" />
		<portlet id="40" />
		<portlet id="79" />
		<portlet id="80" />
	</category>
	<category name="category.cms">
		<category name="category.alfresco">
			<portlet id="91" />
		</category>
	</category>
...
<display>

The name of the category must be a key defined in the resource bundle of the portlet. The value of the id attribute must be the portlet-name as defined in the portlet.xml file. Categories can be nested and default portal categories can be used to add the portlet along with the bundled portlets.