This information is part of the Modelica Standard Library maintained by the Modelica Association.

The discrete state space block defines the relation between the input u and the output y in state space form:

x = A * pre(x) + B * u
y = C * pre(x) + D * u

where pre(x) is the value of the discrete state x at the previous sample time instant. The input is a vector of length nu, the output is a vector of length ny and nx is the number of states. Accordingly

A has the dimension: A(nx,nx),
B has the dimension: B(nx,nu),
C has the dimension: C(ny,nx),
D has the dimension: D(ny,nu)

Example:

parameter: A = [0.12, 2;3, 1.5]
parameter: B = [2, 7;3, 1]
parameter: C = [0.1, 2]
parameter: D = zeros(ny,nu)

results in the following equations:
  [x[1]]   [0.12  2.00] [pre(x[1])]   [2.0  7.0] [u[1]]
  [    ] = [          ]*[         ] + [        ]*[    ]
  [x[2]]   [3.00  1.50] [pre(x[2])]   [0.1  2.0] [u[2]]
                             [pre(x[1])]            [u[1]]
       y[1]   = [0.1  2.0] * [         ] + [0  0] * [    ]
                             [pre(x[2])]            [u[2]]