A content zone is associated with a Java class (actually the interface com.splwg.base.web.portal.IPortalZoneHandler) that is responsible for creating the zone's HTML. When a portal needs to be rendered, the server instantiates a new handler instance for every zone for the request. In principle the handler could do anything within the bounds of the J2EE architecture to create its content. In practice, the vast majority of zones need to make a service call and create HTML from the result. Fortunately the SimpleZoneHandler has been designed to make this easy, and uses XSLT to perform the transformation from the result data (as an XML document) into HTML. You will usually not need to implement your own handler classes.