Whilst part of the FidelityFX features was an earlier rescaling algorithm, that optionally supplanted FDev's own; CAS, specifically, is literally a sharpening image filter,which increases contrast locally -- it was also active even if you did no upscaling. Load a picture into any paint program, apply a sharpening convolve operator to it, and that's it, basically, albeit a somewhat simpler operation.
FSR also does a sharpening pass after upscaling, but FDev does not currently seem give us an option to set its strength... Maybe it is available somewhere in the setting files... :7
EDIT: I, too, rather liked the way CAS made the terrain pop - brought a lot of "3D" to it. :7 I think my biggest problem with Odyssey terrain, is how much of it looks like heavily aliased, random-like monochrome noise -- patch 5 had ameliorated that to a great degree, but patch 6 made it worse again...