MOCASSIN 2.02 manual
This keyword will cause the results in
ionratio.out (see output files) to be integrated over only those cell that fall under the projection of a slit aligned along the z-axis of the grid. The slit x and y dimensions (in [cm]) are defined by
real2 have value 0. or if they are omitted, no slit is used
and the results are integrated over the whole active volume.