The only CPP key coded in OASIS3-MCT and that can be specified in CPPDEF in make.your_platform file is TREAT_OVERLAY that ensures, in SCRIPR/CONSERV remapping (see section 4.3), that if two cells of the source grid overlay and none is masked a priori, the one with the greater numerical index will not be considered (they also can be both masked); this is mandatory for this remapping. For example, if the grid line with i=1 overlaps the grid line with i=imax, it is the latter that must be masked; when this is not the case with the mask defined in masks.nc, this CPP key forces these rules to be respected.