Why is zscale a required argument? Is it nonlinear - if you scale before vs after does it make a difference?

I ask because I postprocess the mesh to scale x/y to something independent of heightmap resolution, and at that time I have to consider the hmm z scaling. It'd be easier for my use case to either pass an x/y scaling as well as z to hmm, or to not pass a z scaling to hmm and do it myself afterwards alongside x/y. But if I have to pass an accurate z scaling to hmm for the algorithm to work well, then that's a good reason for it to be the way it is.