To allow mix two or more component sets in the same ZUML page, ZK uses XML namespaces to distinguish different sets of components. For example, the namespace of XUL is http://www.zkoss.org/2005/zul, and that of XHTML is http://www.w3.org/1999/xhtml.

On the other hand, most pages uses only one component set. To make such pages easier to write, ZK determines the default namespace based on the extension. For example, the xul and zul extensions imply the XUL namespace. Therefore, developers need only to associate ZUML pages with a proper extension, and then don't need to worry about XML namespace any more.