The concept of Desktop Tree is very simple. If shows you which window belongs to which desktop. And you can drag a window to a desktop to move it there. It looks like the following:

Here, you can see that there are two desktops, one called GLOBAL, and the other called DESK-1. There are no window in the GLOBAL desktop, while there are more than five windows in the DESK-1 desktop. You can drag a window from DESK-1 desktop to GLOBAL desktop, and that window will be made global.

Global - When a window is global, it will appear on every desktop. Some other software call it "sticky window". You can un-globalize a window by moving it into a desktop.