It is explained a little further up!
Furthermore the EFIS screen resolution depends on the Post Process resolution (the SteamVR SS value) and not on the Render Scale resolution (the TAA Render Scale value) I believe this is a bug in VR. In effect, in 2D it seems to depend on the TAA Render Scale only.
The idea is to maximize the gain thanks to the superior CAS Shader. The performance gain (per displayed pixels) using TAA 60 (vs TAA 100) is higher with a higher Post-Process Res because the ratio between the CAS Shader process pixels vs the Rendering Shader pixels is higher and quadratic. In effect, it takes a tiny amount of pixel shader time to upscale the image whereas it can take 100 times more to rendering the same pixels at the upscale res.
Because the CAS Shader is in effect “creating details” it is compensating to some extent and you can counter-balance the loss with a certain ratio TAA/SS for a specific HMD panel resolution. With the Valve Index it is really doing wonders at these settings.
I’m preparing a similar discussion for the Reverb G2 and with it I’m torn between TAA60+SS100 (which is 3172x3100) and TAA110+SS50 (which is 2244x2192). In both cases I have stutter free 30fps, the former is a good overall balance, the later is sharper on EFIS (exactly the same type of cost/benefit I have with the Index)*
It will take you less time to read it all than me writing it up.
*this requires using the latest OpenXR Dev Tools in order to change the OpenXR “Recommended Resolution” to 50% (2244x2192). NB: this is what shows as Post-Processing Resolution in FS2020 fps window.