[en] Based on experimental data it is shown, for some chosen alloys and compounds of iron, that there is no unique relationship between the ⁵⁷Fe-site magnetic hyperfine field, B_hf, and the magnetic moment per Fe atom, μ. Instead, the B_hf-μ plot consists of several branches, each of them being characteristic of a given alloy or compound. Consequently, the effective proportionality constant (hyperfine coupling constant) depends on the alloy system or compound, and for a given alloy system or compound it depends on the composition or even on the lattice site. Consequently, the scaling of B_hf into the underlying μ cannot be done a priori.