A compositing manager is an X client that uses the Composite extension to redirect all windows to offscreen pixmaps, and the Damage extension to track when painting occur on those offscreen pixmaps. It is the responsibility of the compositing manager to paint the pixmaps on the screen, possibly adding effects like translucency or deformations.
This section specifies interactions between compositing managers and applications.
For each screen they manage, compositing manager MUST acquire
ownership of a selection named _NET_WM_CM_Sn, where
n is the screen number. Compositing managers MUST
comply with the conventions for "Manager Selections" described in
section 2.8 of the [ICCCM].
The WM_TRANSIENT_FOR property is defined by the [ICCCM].for managed windows. This specification extends the use of the property to override-redirect windows. If an override-redirect is a pop-up on behalf of another window, then the Client SHOULD set WM_TRANSIENT_FOR on the override-redirect to this other window.
As an example, a Client should set WM_TRANSIENT_FOR on dropdown menus to the toplevel application window that contains the menubar.