A ZUML page might include another ZUML pages directly or indirectly. Since these pages are created for serving the same URL request, they are collectively called a desktop (org.zkoss.zk.ui.Desktop). In other word, a desktop is a collection of pages for serving the same URL request.
As a ZK application interacts with user, more pages might be added to a desktop and some might be removed from a desktop. Similarly, a component might be added to or removed from a page.
Notice that both pages and desktops are created and remove implicitly. There are no API to create or remove them. A page is create each time ZUML loads a page. A page is removed when ZK finds it is no longer referenced. A desktop is created when the first ZUML page is loaded. A desktop is removed if too many desktops are created for the specific session.
The createComponents method in the org.zkoss.zk.ui.Executions class creates only components, not page, even though it loads a ZUML file (aka., page).