If more than one FLT mask is specified (for instance, if values are supplied for both parameter "flt.zero_lowhits" and parameter "flt.zero_snr" ), then they are combined into a single mask. If flt.zero_union is true (i.e. non-zero), then the source region in the combined mask is the union of the source regions in the individual masks. If flt.zero_union is false (i.e. zero), then the source region in the combined mask is the intersection of the source regions in the individual masks. [1]

MAKEMAP, CALCQU