The null elements in the system matrices are a result of the particular formulation. A21 and B2 are the appropriate null matrices since that the disturbance process is not driven by either the states of the plant, P, nor by the control input. C2 is a null matrix because that the disturbance is not directly observable. Only the effects of the disturbance on the plant are directly measureable. The triplet [A11, B1, C1] represents the completely controllable and completely observable open loop plant.