ohhhkaaaaay… maybe I need to correct myself here! Check out the first few posts here: DLSS + UserCfg.opt: there is SecondaryScalingVR and it works!
…it is weird though as recently I was diagnosing something where I was checking the changes in UserCfg after changing the sliders in the sim, and that value was definitely tied to the TAA factor (as in 0.8 was 80% there, etc)!
…unless I am mistaken for PrimaryScaling. 
Oh, yes! I remember the chat I had now and found it, and it WAS PrimaryScaling = TAA. Sorry for misleading info!!
As for official (or unofficial) explanations, we wish! The above link is probably as good as it gets!?