do you mean -
(I wondered what you meant and found this -)
the importance of relativistic effects which are expected to become dominant either at higher (XUV) frequencies or at ultrahigh intensities. Indeed, relativity has to be taken into account in order to properly describe multiphoton processes occurring when high-Z atoms or ions are submitted to an intense XUV radiation pulse. Relativity comes also into play when a light atom is submitted to an ultra-intense laser pulse, since the ejected electrons can acquire a quiver energy comparable to their rest mass energy.