I noticed this value on STEP and GamerPoets video "Skyrim : ini Tweaks : Part 2 : Works for All".
I already posted to GamerPoets and he advised I mention this value to STEP as well.
fOverShoulderAddY=0.0 isn't a value that actually works. You can change it to -500 or 500 and it won't do anything. If you want the camera defaulted to be zoom away/back from your player a bit, you can use fVanityModeMinDist=155. 155 is default I believe, so you'll have to use higher number to zoom the camera out, such as fVanityModeMinDist=255. All this actually does is push the camera back and makes it so that value is as close as your camera can get to your player. Once you hit that 255 point, the camera won't be able to get any closer to your player.
I found this value from a mod on Nexus called Customizable Camera.
I use a controller and wanted my cameras main default camera position (3rd person) further back from player. I don't like how close it always gets. I was able to obtain the perfect position for me with STEP and Customizable Camera .ini tweak values.
Here are more [Camera] tweak values if anyone wants to play around with them without using/adding another mod/.esp.