In the Rashba–Edelstein effect the spin current is a consequence of the spin accumulation that occurs in the material as the charge current flows on its surface (under the influence of a potential difference and, therefore, of an electric field), while in the inverse Rashba–Edelstein effect the spin current is the quantity injected inside the material leading to a spin accumulation and resulting in a charge flow localized at the material surface. In both cases, the asymmetry in the charge and spin current dimensions results in a ratio which dimensionally has the units of a length: this is the origin of the name of this efficiency parameter.