This video explains the difference between coplanar and non-coplanar X-ray diffraction.
Both methods can be used to limit the penetration depth of X-rays into the sample and are good for studying thin layers of material in grazing incidence diffraction (GID). With non-coplanar diffraction, layers down to a few nanometers can be analyzed. This method is also called in-plane grazing incidence diffraction (IPGID).