in the case of the NBWPP, which does also occur in the collision of a real photon and a laser field. Again, we limit in particular to the strong-field limit corresponding to $\xi\gg 1$ at fixed invariant parameters $\varkappa_j\sim 1$. The real electron and positron created via the NBWPP at $\xi\gg 1$ are already ultra-relativistic and a large net number of laser photons of the order of $\xi^3$ are absorbed from the laser field in order to fulfill energy-momentum conservation [@Ritus_1985]. On the other hand, a refractive QED process occurs via a virtual electron-positron pair and this manifests itself in the appearance of the integrals in $ds_1\cdots ds_N$ in the partial amplitude $M^{(1)}$. At larger and larger values of the electric field amplitude, the effective integration region in $ds_1\cdots ds_N$ reduces accordingly, in such a way that the function $F_{\phi}(\phi+\Phi,\{s\})$ is always of the order of or less than unity, and then that the net number of laser photons exchanged is of the order of unity, too. More specifically, we recall that if $p^{\mu}=(\varepsilon,\bm{p})$ is the momentum of a classical electron at the initial value $\phi=0$ ($\bm{A}(0)=\bm{0}$), then the component $p_{\phi}(\phi)$ of the four-momentum $p^{\mu}(\phi)=(\varepsilon(\phi),\bm{p}(\phi))$ at $\phi$ is given by 