All of the uncertain system classes (uss, ufrd) may be multidimensional arrays. This is intended to provide the same functionality as the LTI-arrays of the Control System Toolbox™ software. The command size returns a row vector with the sizes of all dimensions.
The first two dimensions correspond to the outputs and inputs of the system. Any dimensions beyond are referred to as the array dimensions. Hence, if szM = size(M), then szM(3:end) are sizes of the array dimensions of M.
For these types of objects, it is clear that the first two dimensions (system output and input) are interpreted differently from the 3rd, 4th, 5th and higher dimensions (which often model parametrized variability in the system input/output behavior).
umat objects are treated in the same manner. The first two dimensions are the rows and columns of the uncertain matrix. Any dimensions beyond are the array dimensions.