 In the context of MHI an alternative program for achieving relativistic covariance has been developed, based on the idea that the actualization rule, expressed in terms of the Casimir operators of the Galilean group in non-relativistic quantum mechanics, can be transferred to the relativistic domain by changing the symmetry group accordingly. In this case the definite-valued observables of a system become those represented by the Casimir operators of the Poincaré group. Since the mass operator and the squared spin operator are the only Casimir operators of the Poincaré group, these then are always definite-valued observables. This conclusion is in agreement with a usual assumption in quantum field theory: elemental particles always have definite values of mass and spin, and those values are precisely what define the different kinds of elemental particles of the theory (Lombardi, Castagnino, and Ardenghi 2010).