3.2.5.2.9.2 Processing Z-Order Sync Information PDU
When the client receives the Z-Order Sync Information PDU, the ID of the marker window MUST be stored in the Marker Window ID (section 3.2.1.2) store.
If a client-side window that is not remoted from the server (referred to as "Window A") is activated, then the client SHOULD instruct the server to activate the marker window by sending the Client Activate PDU (section 2.2.2.6.1) to the server with the Enabled field set to zero.
Furthermore, if the server instructs the client to activate a RAIL window (referred to as "Window B"), the client SHOULD check the position of Window B with respect to the marker window in the WindowIds field of the Actively Monitored Desktop (section 2.2.1.3.3.2.1). Window B SHOULD only be activated if it appears before the marker window in the WindowIds field of the Actively Monitored Desktop Order.