I haven't yet fully understood what "Custom Parallax" in the context of "Cube Maps" means. Why doesn't it work with the upright standing cuboid in the background?
The vague and often very technical docs are also something I'm often struggling with in the official Blender Online manual. I need to know what something is usefull for and I find illustrations and descriptions of how things work better than abstract mathematical formulas. As soon as I find more time for this I will drill the Blender programmers with questions 😉😁!
I have been learning about parallax mapping lately and might be able to answer your question. It is a long shot but who knows?
Depending on the node stup, your texture can be locked to another axies. You can make a setup appear on the y axies, but not on the x axies!
duerer Not technically sure what this does, however it is 100% necessary to make reflections on spheres in EEVEE match Cycles, and it also makes the reflection move (parallax motion) when you pivot around it, rather than just being painted on the surface which looks fake. I enabled "Custom Parallax" for the reflection cubemap in Blender and left its radius at the default. I only adjusted the reflection probe's settings to specify the radius and clipping start of the probe itself. However, as you lower the custom parallax radius, the reflection has less and less movement and eventually will look the same as if you turned the setting off. Basically, switch to Cycles, look at what your reflection looks like, than switch back to EEVEE and adjust the custom parallax radius if necessary (for my scene, I left it at the default. Don't forget to hit "Bake cubemap only" under Indirect Lighting in the Render tab to see your changes take effect (or turn on Auto Bake).
Thank you very much for your explanation, Adam copperplate 😀! This makes sense to me!