I think @rpedruzzi 's suggestion to create your GRIDMASK file using the spatial allocator setup and scripts was meant for cases where you or someone from your group already have an existing shapefile describing the regions you want to use in your CMAQ-ISAM run. There probably aren’t too many people in this forum who have expertise in creating shapefiles from scratch, so if you don’t have an existing shapefile, you likely are better off creating your GRIDMASK file through tools like m3mask or m3fake as discussed in the other thread you started.