What is the beam propagation direction?

[jhuang97]

In Kirkland's computem / temsim, the electron beam propagates in the positive z direction, if I recall correctly.

I notice here that in src/PRISM01_calcPotential.cpp, the z coordinate of the atoms seem to be negated at one point, with the lines
return round((-t_z + pars.tiledCellDim[0]) / pars.meta.sliceThickness + 0.5) - 1;
and
return (-t_z + max_z);

Why is it like this? As a consequence of this, does the electron beam propagate through the slices in decreasing order of z-coordinate in Prismatic?