The relative weight, delta(Beta), of the yield of secondary electrons, SE(2), induced by the backscattered electrons, BSE, with respect to that, delta(P), of secondary electrons, SE(1), induced by the primary electrons, PE, is deduced from simple theoretical considerations. At primary energies E(0) larger than E(M) (where the total SE yield delta = delta(P) + delta(B) is maximum), the dominant role of the backscattering events is established. It is illustrated in SEM by a direct comparison of the contrast between SE images and BSE images obtained at E(0) approximately 5 keV and E(0) approximately 15 keV on a stratified specimen. At energies E(0) less than E(M), the dominant role of SE(1) electrons with respect to SE(2) (and SE(3)) is established. It is illustrated by the better practical resolution of diamond images obtained with an in-lens detection in low voltage SEM E(0) approximately 0.2-1 keV range compared with that obtained with a lateral detector. The present contribution illustrates the improved performance of LVSEM in terms of contrast and of practical resolution as well as the importance of variable voltage methods for subsurface imaging. The common opinion that the practical lateral resolution is given by the incident spot diameter is also reconsidered in LVSEM.