The 6 Probability Maps Normalization is based on PMOD's Java implementation of the Unified Segmentation methodology in SPM12 developed by Ashburner et al [11]. In addition to the probability maps of grey matter, white matter and CSF it uses probability maps of bone, soft tissue and air/background.
In the user interface the 6 Probability Maps Normalization is configured as the Probab method and activating the preset so that is marked by the red bar.