Although the very important concept of relative order is defined in differential geometric control with respect to an affine input-state-output (I-S-O) system, it is rather the difference between the highest orders of the derivative of output and that of input in the corresponding input-output (I-O) systems. This is a direct generalization of the result in the case of the transfer function in linear control theory. The relative order is proved to be an invariant of realization, and applications in realization and minimal equivalence are discussed.