Update 04-conceptual-design.md about planes.

documentation/design/04-conceptual-design.md

@@ -61,3 +61,18 @@ Some conceptual building blocks for a network supervisor:
 * **Northbound Interface (SBI)** 
 
 * **East-West-bound Interface (SBI)**
+
+
+## Applying Changes to What Plane?
+
+Some basic thoughts to dissect how different approaches are applying changes to the various planes. 
+
+### Changes to the Control Plane
+
+### Changes to the Data Plane
+
+This is the use case for the SDN approach: A so-called SDN-controller applies policy rules to the data plane. These policy rules are defining the handling of the flows in the networks on a larger scale or to be more precise the handling of more less specified packets. 
+
+A change to the data plane will not directly trigger a change to other planes. Though the flow of packets on the data plane can be observed by the control plane and the control plane can take action depending on the data packets. 
+
+### Changes to the Management Plane