The system tray is an X client which owns a special manager selection on a given screen and provides container windows.

The selection owner window is the window belonging to the System Tray that owns the manager selection (as in XGetSelectionOwner()/XSetSelectionOwner(). Note that this probably is not the same window that's used to contain the system tray icons.

The tray icon is a window to be embedded in the system tray.