If you have multiple local structures in your sample, the XAFS signal is a simple average of those local structures, weighted by the probability that an atom with a particular structure absorbs an X-ray. For a uniform sample with a mixture of sites, that weighting is the simply the fraction of atoms with each structure.   If the sample is not uniform, it can be more complicated.  In particular, heterogeneity that is a function of depth (say, a layered material) is more complicated, because atoms near the surface are more likely to absorb the X-ray than samples at depth.