Top: Final region map used in the sampling procedure for the synchrotron spectral index. Middle: Planck 2018 polarized synchrotron amplitude map with the corresponding spectral index processing mask for BEYONDPLANCK and a region outline that divides the sky into six disjoint regions. This was further reduced to four, motivated primarily by the low S/N apparent in some of the regions. The mask is chosen to reduce temperature-to-polarization leakage bias, and to exclude bright point sources. Bottom: Planck 2018 polarized thermal dust amplitude with the corresponding processing mask.