The article deals with two issues concerning reflection electron energy loss spectroscopy (REELS), namely, which angular cutoff should be used to properly define the phase space of energy loss into plasmon excitations and how approximate evaluations of the surface component of the momentum transfer can affect the surface excitation parameter. With regard to the first point, we demonstrate the crucial role of plasmon energy dispersion in determining the angular range for inelastic scattering. As for the second point, we show that an exact evaluation of the surface component of the momentum transfer is needed if the surface excitation parameter has to be determined in a reliable way over the entire range of angles of surface crossing.