Spin-polarized X-ray photoelectron diffraction (SPXPD) spectra are useful to study magnetic structures of solid surfaces. In this work two different SPXPD modes are studied on the basis of a relativistic quantum electrodynamic (QED) theory which provides a unified picture of the SPXPD analyses. An important advantage of this theory is to allow us to handle many-body relativistic XPS theory, which can be a basis of SPXPD analyses. One mode is to utilize the spin polarization caused by multiplet splittings as observed in O₂ molecules and Gd metals. We demonstrate how to extract local magnetic order in surface O₂ . The second SPXPD mode is to use circularly polarized X-rays to discriminate the spin polarization with aid of spin-orbit coupling. [DOI: 10.1380/ejssnt.2005.353]