For a multi-output ARX structure with NY outputs and NU inputs, the orders [NA NB NK] define the model structure as follows:
NA:
an NY by NY matrix whose i-j entry is the order of the polynomial (in the delay operator) that relates the j'th output to the i'th output.
NB:
an NY by NU matrix whose i-j entry is the order of the polynomial that relates the j'th input to the i'th output.
NK:
an NY by NU matrix whose i-j entry is the delay from the j'th input to the i'th output.
The Order Editor allows the choices: where na, nb, and nk are chosen by the pop-up menus.

For tailor made choices, construct a matrix [NA NB NK] in the MATLAB command window and enter the name of this matrix in the Orders: edit box of the Parametric Models dialog.

Help topics.

(file mvarx.htm)