𝜆 denotes the relative position in the EDL where the electron transfer occurs (Eq. 10a): At 𝜆 = 0 the transfer occurs so close to the electrode that the electrostatic potential at the reactant and product systems is equal to that within the electrode itself. This case is depicted in Fig. . At 𝜆 = 1 the transfer occurs at a position where the electrostatic potential is not affected by the electrode potential. Here 𝛿 𝑖∥ ⊖ = 𝛿 𝑖𝑎𝑞 ⊖ and the change in electrochemical Gibbs free energy with electrode potential stems only from changes in the Fermi energy of the electron in the electrode. The meaning of 𝜇 is more obscure (Eq. 10b): It describes how close the relative effective charge is to the reductant state (𝜇 = 0) and the oxidant state (𝜇 = 1). Intuition suggests that this value, like the symmetry coefficient 𝛼 of the BEP principle, should be close to 0.5 where the relative effective charge is equally different from both systems. But after all, the effective charge is only capturing the change in Gibbs free energy of the system when the local potential varies (for point like systems), or the electric field varies (for spatial extended systems). Additionally, 𝛿 𝑖∥ ⊖ is not necessarily constant over such wide potential regions and conditions as can be probed in electrochemistry, hence 𝜇 likely varies with potential.